I don't think Angel ever killed a slayer, because why would ME pass up that dramarama narrative? But the narrative answer for me is that Angel probably wouldn't have gotten as obsessed about corrupting a Slayer (since it seemed like he was fond of horrifying/devastating/super-stalking people who were resisting victims, not girls who came out fighting. And I think Darla loved to live large, which for most of her demonic time focused less on having challenging fights than it did on living in nice places and wearing pretty clothes and having pretty things (*cough* Angel *cough*).

By the way, you might be interested in the different Buffy Trivia guides/sites. There's lots of helpful, basic info at those places (especially for things like body counts, who turned whom, references to other characters, even references to characters' hair!). The Buffy Dialogue Database is also good for this kind of thing, checking hunches or finding details if you have an idea of at least what season it might be, and also has a search feature that's very helpful.

Is it just me or do random references to Angel seem to increase in this season?
I think it's you. :) IIRC, there are far more references to him in s4. But again, you could check the Dialogue Database to see if it bears out.