This can happen. Kel'Thuzad delights in dragging heroes to the darkside. A "will" is all that really prevents him from taking complete control of someone, but will can be shaken and broken. It's just a matter of doing it.

As far as being pissed off goes, I think it's fair to say that his sole mission in unlife is to make the entire City hate him anyway. So yes, we should plot.