Oh, now this is an interesting thread. Psy and Teddy pre-TIRADE!

I think that Percy acted like such a jerk because, well, think about it; He'd strove for power, preferably at the Ministry, pretty much his whole life, and then the second he gets a job there, his boss goes slightly insane and is killed, he gets into a ton of trouble, but then he gets a promotion. Admittedly suspicious, but he'd wanted a job like that his whole life, so he turns a blind eye to it. One thing leads to another, he's fighting with his family. Then, finally, he realizes that he was a "Ministry-loving, family-disowning, attention-seeking prat", he's 100% upset and sorry about it, and he's on the right side again.
At least he admitted he was wrong before it was too late...