The information has been outdated after the patch 2.4.0. It is retained only for historical reasons.
M'uru Martyr Theory Edit
There is some questioning over whether M'uru is truly a prisoner in Silvermoon City or if he chooses to remain there of his free will. The Naaru are very powerful creatures, in some cases at least as powerful as the strongest of the Eredar warlocks, raising a question mark over M'uru's capture. It also seems odd that M'uru was the only Naaru in the entirety of Tempest Keep. Although the procedures of how the Blood Elves claim to keep M'uru in bondage are still relatively unknown, some theorize that M'uru purposely allows his knowledge to be sapped under the belief that he could change and redeem the Blood Elves.
Despite the existence of counter arguments, the words of the most powerful of the naaru, A'dal in his discussion with Lady Liadrin illustrates that he himself, as well as M'uru were aware that M'uru would be captured, or at least would suffer some kind of misfortune ahead of time. A'dal also suggests by his words that "M'uru was aware of his fate, long ago", giving a hint that they were aware of what was going on and that M'uru was fully aware of what would become of him.
Counter to Martyr Theory Edit
This theory does have a number of holes in it, however. The most glaring issue is what individual Naaru are really capable of. If M'uru's power were on par with individuals such as Kil'jaeden or Azshara, then there is no way Kael's Blood Elves could have subdued him. On the other hand, if M'uru's personal powers were at the same level as, say, a dragon, then Kael's elves would have been able to subdue him with a bit of effort.
Additionally, the text of the Blood Elf Paladin quest  states that there has been trouble concerning M'uru, whether this has anything to do with him trying to escape from custody in Silvermoon (damaging some of the Magister's equipment in the process) or not is unclear. If M'uru was indeed in Silvermoon voluntarily as is claimed, then it is very unlikely that he would be trying - apparently violently - to escape unless he was a particularly dedicated actor.
Counter to Counter to Martyr Theory Edit
However, it seems odd that the Sha'tar are so hospitable to the Blood Knights and have a portal to Silvermoon, yet make no attempt to rescue M'uru.
Counter to Counter to something based on Fel Reaver SmackdownEdit
Keep in mind that YouTube videos are floating around of the Hellfire Peninsula Fel Reaver having been kited all the way back to the Terrace of Light in Shattrath City, and A'dal one-shot killed him for 170,000+ damage. As Blood Elves lack the fortitude of a Fel Reaver, it stands to reason that M'uru is a willing guest of the elves. But then again, a gimpy Orc with missing body parts captured and enslaved Alexstrasza once, so anything may be possible (although said "gimpy orc" did have in his possession the Demon Soul, and as such had an emmense amount of power, especially over dragons).
M'uru Faster Reincarnation Theory Edit
An alternative theory would involve the Naaru cycle of rebirth. Take for example A'dal that is hailed as the most powerful being you (the player) will likely ever see and you see a shining beacon of holy light. Other Naaru are colored darker and emit less light (there are various degrees of this). M'uru appears with a very dark color and he emits very little light (although this could be a physical reaction to being held captive by blood magic).
Based on this evidence, it is possible that M'uru is very close to darkness and therefore significantly weaker then the other Naaru. This conclusion makes it a lot more plausible that Kael'thas was actually able to subdue M'uru and that the Blood Elven magisters are able to keep him restrained. It would also make sense that A'dal left M'uru at the Tempest Keep for this same reason, his weakened state making him unable to stand against the Illidari and Burning Legion.
However, this leaves one or two questions left to be answered, "Is M'uru a prisoner or martyr? If he is a prisoner, why is he not rescued?"
It's safe to say that A'dal is aware of M'uru's state since some of the Scryers, that literally live on his doorstep, were Kael'thas' best and brightest, it is likely that some would have been included in the conquest of the Tempest Keep, and perhaps were even directly involved in the capture of M'uru. Though A'dal has not yet taken action, it would be difficult to prove that he is unaware of his compatriot's condition.
It is likely that the siphoning of M'uru's holy power causes him to fall into a shadow state faster then normal. And as some of you may know, a fallen Naaru may be 'revived' with a proper source of holy power, something the other Naaru are not in short supply of. So, if the Blood Elves help to speed up M'uru's decline into shadow he may be revived sooner and be reborn into a Naaru's full glory. Currently A'dal is the closest to this, but a newly reborn Naaru may be many times more powerful, a formidable force in the war against the legion and the Illidari.
M'uru Control Theory Edit
Also, the Holy Light in itself is very likely to hold cleansing properties, therefore the simple wielding of Holy power may contribute to cleansing the corrupting influence of their prince. Lastly, the Holy powers gained from M'uru's imprisonment allow the Blood Elves to make a stand against the Scourge, which is in itself part of the Burning Legion. In essence that also advances the Naaru's goals.
The Naaru may simply be using a subtle form of mind control on the Blood Knights. If fel energy can affect the mind and body in a radical way, it stands to reason that holy magic being subtle may be able to affect Blood Elf body and mindset. They may become addicted to the Naaru's energies, replacing the Well of Eternity. This would create a battle hardened fighting force for the Naaru, and allow them a voice in Horde politics albeit in underhanded way. M'uru might have a spell similar to the Priest spell Mind Control, only more passive. M'uru may simply slowly be moving the Blood Knights as pawns for the Army of Light. They also can easily be pawns since the Alliance would see them as traitors at best, while the Horde would see them with pity, and leaving them with few options but to serve higher powers able to sate their addiction. This would allow them a way to control the Blood Elves. They may tolerate M'uru's imprisonment as he is the ultimate spy or mole in the Horde organization since one of the leaders in Silvermoon is a paladin.
M'uru Naaru Nuke Theory Edit
In a darker note, a Naaru who dies like D'ore "becomes a shadowy void which attracts and consumes souls." D'ore in the quest  explains this void: "For several centuries, the spirits of Auchindoun coalesced into my darkened essence. Many of the catastrophes that befell this crypt were a direct result of my weakened state." The voidwalkers of Auchindoun and Oshu'Gun are physical manifestations of this curse. This could mean like a suicide bomber, M'uru' is speeding up his life cycle to become like A'dal. The Night elves will get a chance to destroy or cripple Silvermoon itself. Even if this attack fails, it would curse Silvermoon, to a similar extent how Auchindoun and Oshu'Gun are cursed. This is not even accounting for the Scourge remnants around the Blood Elven lands, who knows what would those Scourge mutate into under a curse that powerful? An attack like this would be devastating to the Blood Elves. This could also explain why the Night Elves are positioning themselves near An'owyn, An'daroth and An'telas. They can attack Silvermoon directly or simply mop up Blood Elf refugees fleeing from Silvermoon. Unlike most species, Naaru are reborn after their death, making such a kamikaze attack more logical than A'dal allowing his ally remain under captivity. The Naaru can also claim that the Blood Elves' own addiction to magic destroyed them. M'uru would become fully regenerated at the cost of the lives of the Blood Elves. To many who would see this to be a conspiracy theory, understand that the Naaru are not above manipulating whole races in their fight against the Burning Legion. In the same way they may be working directly (through the Draenei) or using subtle means (a large chunk of the Alliance) to create their Army of the Light. They may be culling the herd, by removing a race that may change to a race similar to the Wretched. It may also send message to the Horde that the Naaru are not to be trifled with. Its true that an "incident" in Silvermoon would be suspicious to the Horde's eyes. Then again it could be seen as an incident of elven magical overreach, like the destruction of the Well of Eternity. It is doubtful that the Horde would strike at the Naaru since clearly they had knowledge of their life cycle. The Alliance could also have plausible deniability, since technically they had nothing to do with this. Alliance spin in the events: "We didn't understand how they made Blood Knights, but we never expected the horrible truth, they deserved their fate for claiming power that was not earned." The rest of the Horde or at least the Orcs know the Naaru life cycle from Hero of the Mag'har quest chain. So they can't really blame the Alliance for this since, clearly they know what happens to a dying Naaru. M'uru "death" would be like blaming an old orc or human of dying of old age. The Scryers know that only by serving the Naaru would their race be saved from destruction. Voren’thal even said (from Worldofwarcraft.com and the Scryer article itself) “I’ve seen you in a vision, Naaru. My race’s only hope for survival lies with you." Clearly total annihilation is the fate of the Blood Elves if they don't serve the Naaru. The enslavement of M'uru would be the antithesis to this idea. This could be seen by the Scryers that Voren'thal was right all along however they would be likely horrified at the tremendous of civilians that would get turned into voidwalkers. However such a brutal attack equivalent to a nuclear bomb (virtually rendering Silvermoon inhospitable) and the ruining of Silvermoon would earn the Naaru many enemies. The Scryers would undoubtedly be infuriated by such an attack on their people and their beloved city and it doesn't explain the presence of Blood Knights in Shattrath.
Counter to Naaru Nuke Theory Edit
Counter to Counter to Naaru Nuke Theory Edit
While the player character Blood Elves and the Scryers are not enemies of the Naaru, we can assume that most, if not all of the NPC Blood Elves leaving Silvermoon are joining Kael's forces. IF this is the case, then it's fairly obvious that the destruction of Silvermoon would cut off Kael's reinforcements, and demoralize the Blood Elves currently under his command.
Counter to the above Counter Edit
The problem with the above counter theory can be learned from the Scryers after completing a certain quest. This is that after the Scryers found out about Kael's allegiance with the Burning Legion, and began telling new Blood Elf pilgrims this information, more of the pilgrims began joining the Scryers. This means that while many of those still on Azeroth are completely faithful to Kael, once they come to Outland and learn of his real allegiance they are more likely to join with the Scryers, and in turn the Naaru.
Important Fact about the Naaru Nuke Theory Edit
If the Naaru really plans to blow Silvermoon out of the map, there are many chances that the remaining Blood Elves, including Scryers and Players, will turn back against the Naaru : destroying Silvermoon is like destroying all the things the Blood Elves are fighting for. Without their people and home to protect, why do the Elves fighting the Burning Legion side by side with the Naaru and other races?
Dark Naaru Theory Edit
Maybe draining has weakened M'uru so much that he turned into a Dark Naaru (he glows dark light, so this is very possible) and can´t leave, due to the fact that Dark Naarus are much weaker that normal Naarus. This would also accompany the fact that in the upcoming Sunwell Plateau Raid, one of the bosses has speculated to be a Dark Naaru, which means that M'uru may be being used by Kael'thas; and those who are draining M'uru are most likely to be followers of Kael.
Now that it has been confirmed that M'uru is a boss in Sunwell, this could lend credence to some of the previous theories. For example, assuming that M'uru was aware of all events as they would unfold due to premonitions, it could have been preparing its suicide tactic to be ready after it has been taken deep into the Sunwell, close to Kil'Jaeden.