But Krista and Janice and co. are just as nasty.
Exactly.
I see it like this: where Amy's more terrible qualities stem from ignorance and a lacking self-awareness, people like Janice & Damon are intentionally malicious, unforgiving people. What they did to Amy in the S1 finale was unbelievably cruel and honestly, far, far worse than any of Amy's shittier moments.
Krista's certainly awful too, but I don't see her going out of her way to be an asshole like those two do.