Actually, the legal consequences didn't even occur to me - I was thinking more along the lines of how James himself would react to having used that curse to kill someone. I think it would really affect him, and possibly those around him as well. If I were exploring such a scenario, I would probably send James off on some soul-searching. First, because he used Dark Magic, which we are told he hated. And second, because he killed someone, which is usually something very difficult to integrate into one's psyche: I think there would be guilt, fear, doubt, self-anger, and more. But I have never done anything remotely similar, so I can't speak from experience.
I'm not so sure the governmental repercussions would be that big; by that point, nearly everyone was in a state of constant terror. If James had killed, say, Rosier (James didn't, Moody did, but let's not worry about that for the moment) and James was able to demonstrate either that Rosier was Marked or was attacking James' family, he'd probably have gotten away with it.