Since I was one of those people, I'll try to field this. IMO, it *is* the cost of doing business that villains will try to end the world if they can. Angelus, Willow, Lex Luthor, most Bond villains, etc, for one reason or another, no matter how asinine, they're out to kill everybody, or at least utterly cow them into submission. Why? Because they're the bad guys, that's why.
Further, Spike claimed that he was in love with Buffy. The fact that this supposed love didn't crop up until he had some ridiculous dream aside, if she had been in her right mind, she never would have gotten involved with him, even sexually. If a monster falls in love with a hero, that's one thing. If a hero returns those feelings, they're not really a hero anymore, IMO. BTVS was never supposed to be a show about gray morality, and it *wasn't* one until Joss decided that Spike was such a special snowflake that he was the only vampire in the history of ever who could decide to get a soul. If he can do it, why couldn't others? Buffy herself was never my favorite character on the show to begin with, but I don't appreciate her retroactively being made into a killer because there was the chance that some vamp somewhere could have opted to reform. YMMV.