Back on the original topic, here's a reckless thought thrown in about why Dumbledore was so trusting of Snape. Maybe Snape took an Unbreakable Vow with Dumbledore? We don't really know anything about how much freedom people have when they've taken a vow or how the vow determines whether you've broken it, but say he was under a vow to do whatever Dumbledore tells him to do? When he's under two Unbreakable Vows, perhaps by Dumbledore telling Snape to kill him since he was dying anyway, he was able to satisfy both vows.
It's a better reason than the flimsy one about Snape being regretful for putting Voldemort on James's and Lily's trail. Just a guess.
