Do I owe anything to my roommates?

First of all, I paid first and last month when I moved in. Second, my roommates let some douche bag sleep in my bed when he was drunk and high. He peed in it, then apparently put my birthday cards on the floor and slept on them instead of in his piss. I would say someone owes me for that, right? Anyhow, I had had enough of the crazy parties with girls throwing up and random people I don't know breaking and stealing my stuff, and moved out. (Not much of a partier.) We paid rent at the first of every month and I moved out half way through May, which I paid for at the beginning of the month. They say I still owe utilities and a month of rent, but considering I paid first and last month at the beginning and rent for the month of May on the first, (also considering my mattress) I would say they owe me... am I right?