Was Darla the First?

Looking back on it, It would make perfect sense that the Darla that appeared to Connor to stop him from killing that girl for Cordelia is the first. To start off, that was the year the first was totally after the scoobies in Sunnydale.

Is the First Evil in Angel?

The First Evil is something that Buffy isn't able to fight off or kill. Angel is convinced by the First Evil, that either he has to either kill Buffy or himself. There is no way for humans to fight this purest form of evil. Angel realizes that, even though Buffy tries to save him.

Why did Angel kill Darla?

She sacrifices herself in order to give birth to her and Angel's human son Connor, ending her run on the series. However, Darla continues to appear in flashback episodes during the next two seasons.