The thing I hate about Billy's character is that he's this kind of person that will always want to pin things on others so that they are going to appear bad and him being righteous as the one that is supposed to be closer to you as your friend.
The trait of someone who is going to be a horrible roommate is what I am going to point out. For me, it will be someone who always feels like he is the most righteous person or self righteous person in the group.