I personally don't think Angelus killed a slayer, the question isn't does he have the skill, obvious thats a yes, but does he have the will? Angelus as we already know was kind of on a higher level, he seemed less monster, but way more superior to human, and I think Angelus liked to get to know his prey alot better and be able to play with it alittle. I think the only reason he messed with Buffy and her friends was the amount of personal things he could bring to it, he knew what would set them off and he used that to his advantage. Which we saw in Passions.
I think he never killed a slayer because he never wanted to. He was way above that, and it would simply be a kill, and more or less I say it would have bored him. Spike gained his merit for killing a slayer, because his brawls and little fights were his forte, but now that he's killed one of the big guns, hes made it into Angelus's grace.