First of all, don't avoid her entirely. That is low as fuck. If you don't want to see her anymore, tell her that, if you think you need a reason, I'd go with the girlfriend or the parents thing, either is reasonable... I think you should find out what she did first, if it's not too serious, then it shouldnt be a big deal, I doubt that she killed anyone. plus you said she seems nice. Maybe she made a mistake or got caught up in someone elses... Do a little research before you jump to a conclusion.
-katie