Percy doesn't really have many admirable traits.
Sometimes Fred would just list all his faults in his head, and when he ran out he still kept going...he likes muggle rock music, he chews his damn cereal too loudly, licks his lips too much and God, it's annoying...
Ridiculous things to qualify as a fault. But then if Fred stopped listing his faults, what would there be left to think about? His only excuse for thinking about him so much in the first place was his dislike and annoyance that something like that could be in his family, could be called a Weasley, and really it was a flimsy excuse. Fred knew it. Shouldn't be thinking about him obsessively anyway.
Betrayed his family, he did. And really, maybe he's not a Weasley. Maybe he was misplaced at birth, and he's not related to me at all. He carries himself differently, his skin has more of an olive tone, and his eyes have brown flecks in them. And his body, oh... Percy looks nothing like a Weasley should. Fred could almost convince himself, but not quite.
It was less easy to convince himself that what he heard whilst evesdropping on the Order, (with George as per usual, of course) was a lie. Percy isn't that noble, he wouldn't spy to save his family. That's number one on the faults list- where would Fred be without that? The Extendable Ears were just a prototype. Must have misheard.
Too much of a git. Percy, Percy is a git.