Gonna keep my thoughts to myself on this one. Except stay in drugs, eat school and don't do vegetables, vice.
Talk to him about God. See if that gets him away frome u FAST! If this person is a true satanist i would think that would work.
Sparrow bro, don't worry about him, just stay on guard. He most probably isn't a true satanist, the notebook proofs are not enough, and in fact, there is a big difference between people who are nutcase and satan worshipers! Nothing to worry about the note because that's not something that appeared in a Satan bible or anything, and the sign is widely known! The sign, if turned in other ways also represented many other things. And no I don't know your roommate so don't ask me that, but I have seen true satanist and that's not how they act. They work more on seduction and life, rather than what people think, blood. And well, don't be a Satanist hater, because that is a RELIGION and please respect that. Just like how you wouldn't like people to be rude against your religion, don't be rude to others no matter how much it might be against yours in terms of ways and acts. Ps I'm not a satanist, I'm a buddhist. Just that I was curious and did many research on various religions. ๎๎