Actually, of all the people Spike killed, Nikki Wood and that Chinese Slayer are the last two he should feel guilty about.

Is it sad that Wood lost his mother, hell yes, was she a good person who had a right to live, hell yes.

But you know what else she was? She was a vampire slayer. She was a warrior in a war who went out intentionally to hunt down and kill vampires.

Fighting Slayers is the closest Spike came to having a fair fight, since Slayers are stronger and more powerful than vampires.

They were both in a war and that night, Nikki lost the war.

It's sad, but as far as Spike's sins go, killing a Slayer is one of the least bad things he did.