It is murder, but you're going to have terrible trouble proving that (especially if he's careful about documenting how fair the russian roulette is). To avoid murder charges, the hypothetical psychopathic death-immune person can go so far as to arrange a tournament, with 2^n entries for some integer n. In this arrangement, one person must survive every round, and thus it does not look suspicious afterwards that he did survive every round (plus he gets 2^n prizes for going through n rounds).

I'm pretty sure that the russian roulette itself is illegal just about everywhere, though. No matter how it's done, it's either murder or assisted suicide.