I was afraid that something like this would happen from the beginning. At least it seems Maxou's death was quick, hopefully painless. If I were you I would never even let my cousin back in my house, let alone room. It was one thing to take something without asking, but to react in such a way when confronted? Instead of simply owning up to it and asking for forgiveness he became violent. What if he had a weapon nearby? He just sounds dangerous.
Remember that the tank your fish lives in is his or her's whole world. Ask yourself this question, and answer it honestly: would you choose to live there?