I just have to say that #5 is so underrated. She's hilarious, and the poor girl is always the one who gets yelled at/blamed
I think it's pretty clear that Pete is not the RD. If he was, why was he only THEN acting all weird and guilty? It wouldn't have made him feel anything if he was part of a big revenge murder plan. I think it's pretty clear that he just murdered someone random, most likely Chad in self defense (or maybe on accident).
I think the contenders are Grace and Hester. I never thought Cathy had anything to do with this stuff but last night's episode made me kind of suspicious.