Also don't get the statement "that's the way the game plays".
So why bother with the rape attempt with such argument - "that's the way the game plays" - Buffy was a Slayer, Spike was a vampire - they were natural enemies, so his attack was only natural thing. Nope, doesn't work even for Spike-haters, I guess.
And again for this "natural enemies" argument - Wood was a vampire-hunter, Spike was a vampire (soul or no soul - still vampire) - the "natural" thing to do for a vampire-hunter is to kill a vampire.
So why exactly the first case with Spike and Nikki is more "natural" than the other two?
Wood went after Spike not because he was a vampire-hunter, he had personal reasons; he lost his mother and grew up with the idea that one day he would find the murderer and kill him. He found him, in the worst time possible, and was used by the First (and Giles).
Buffy and Spike had a non-relationship, it was a personal thing.
Now, evil vampire x powerful Slayer is part of a game; like that American Sniper movie: it seems to me (I haven't watched it) that the guy is considered a hero. He killed hundreds of people. The guys from the other side are probably considered heroes, too, having killed many Americans. Not a good thing, though, from both sides, to brag about it, but they do, because is part of the “game”. Spike liked a brawl, so he sought powerful opponents; it wasn't cold-blooded murder.