Reb 80,
He is asking a theoretical question. Granted that we know that he is doing it maliciously, would there be a Mitzva to save him? Your second point is, of course, true. There is a Mitzva not only to save his life, since he’s is not yet a Rasha, but to save him from commiting an act that will make him a Rasha. This is actually worse than the regular Lo Saamod because you are supposed to stop him from doing an Aveira.