Originally a noble and talented soldier from Stormwind, he led an expedition from Stormwind to Stranglethorn Vale. Unfortunately, sometime during the expedition he fell victim to foul magic; his mind becoming possessed by Mai'Zoth, a powerful Ogre mage of local Mosh'oggOgre clan, seeking to use Kurzen and his men for their own deeds.
Although the majority of Kurzen's men still follow their leader, a small group of his men, troubled by this sudden change in Kurzen, rebelled and estalished their own group at the Rebel Camp. Unlike Kurzen's force, the rebels loyalties still lie with Stormwind. Lieutenant Doren, leader of the rebels wants Kurzen's head to bring an end to his madness.
Colonel Kurzen is a reference to Apocalypse Now. Kurzen, who has gone "insane" and has an army to fight for him from a small, hidden compound, parallels Colonel Kurtz, played by Marlon Brando. The two Colonels' backstories are very similar. It is, by extension, a reference to Kurtz from Heart of Darkness, the Joseph Conrad novella upon which Apocalypse Now is based.