I wouldnt be surprised TBH
In the tomb of sargeras audio drama there were some heavy hints at it, and khadgar did sound quite scary the way he was hunting guldan.
Kiljaedin specifically told guldan to not kill him, they have plans for khadgar at any rate, whether he be a tool, an agent or whatever.
The temptation in kharazan is a telling sign that the legion wants him, kharazan is going to be a mega dungeon come 7.1, we will likely see something big happen then as to whether khadgar gets tempted further or not.
Remember arthas was a paladin, he did everything to protect his people, and ultimately did everything to destroy them, he snapped in northrend, medivh was a protector and guardian too lest we forget, khadgar is strong but it seems the legion is intent on using him some way.
Honestly i wouldnt be surprised if he does becomea villain, whether he snaps, takes the power of the guardian to fight a crucial battle that we would inevitably lose, drives the legion back but winds up becoming an unwitting avatar of sargeras or something.