Okay I have my answer now. I just went to Karador and killed him but nothing happens... Quest log doesn't update and I can still pick up a fight with him (but the battlefield is empty and I win instantly).
I guess my game is completely broken now and I'll have to wait for the next patch.
*sight*
I wish I had bought a retail version instead of the steam one so I could revert to the 1.6 version...
