All who knows something of the WaCraft's lore will know too something BAD!
Well, it's not soo bad, it's only a mistake... xD
Before The Burning Crusade:
The Eredar, some BAD BAD BAD guys, demons that kill worlds, eat children, steal cookie...
Well, they were so bad.
One day Sargeras was ordered to give them a lesson.
Sageras jailed them in the Twisting Nether, but he become a bit evil...
Who doesn't resist to robe some cookies, uh?
Later he discovered the Nathrezim, who where so bad too.
They would be the last ones in being jailed by Sargeras.
He become a Demon, he free the demons, he created an army name the Burning Legion and he become more Evil than all the demons.
So, Sargeras was GOOD and the Eredar BAD.
After The Burning Crusade:
The Eredar was a happy happy happy race, living in their planet, Argus.
One day a very bad guy with a Burning Legion reached Argus: Sargeras.
He corrupted most of the Eredar, but one of his leaders, Velen, escaped with the aid of the naaru with some loyal Eredar.
They scaped and scaped and finally reached a world. They called the world Draenor and they called themselves Draenei (how a imagination xD )
So, Sargeras was BAD and the Eredar GOOD.
After these funny-related history, all of you should think abou it. WTF?
Retcon... Or no... If we have the neede imagination :D
After Lon-ami have an idea
1- Sargeras good, Eredar good.
2- Sargeras good, Eredar bad. We can suppose Eredars become bad by a fel way. Things that happen when playing with magic.
3- Sargeras jails the Eredar, and become a bit evil.
4- While Sargeras jails another demons, the Eredar recover from their evilness.
5- Sargeras go mad, and free the demons. The Eredar had become good, because they were the first ones in being jailed (We can suppose, if a fel energie exist, that a holy ones exists too, and Sargeras give his soul's holy energies for the recovering of the Eredar)
6- Some of the Eredar become bad again. Velen knew the things they suffer while they were demons, and escapes with the aid of the naaru.
I think it's a good way of conserving the both histories ;).