Maybe she had feelings for him, that she wasn't happy to acknowledge, - but kept her from finishing him.

The Buffy and Spike dynamic was my favorite subplot because of how complex and developed it was. I think they always kind of had a thing for each other. I didn't see it as love; more like fascination/obsession and then eventually friendship and trusted confidants. Even as far back as School Hard, they always had a certain underlying chemistry even when they were fighting. Buffy probably didn't kill Spike because she saw a smidgen of humanity in him even when he was evil.

@tv_is_my_parent said:

No reason is enough. She had plenty of oppurtunites to kill him before he became her lover.

I agree. Spike should have been dusted. Unfortunately he stuck around to be a pain in the ass. Don't get me wrong, I liked him as a villain & he could be funny at times but spuffy was a no no for me. I stopped watching the show after that.

You could say that for Dracula, Dru or any other vamp that got away. Popularity by fans kept bringing them back