In your campaign how do you handle the redemption of creatures and NPCs? I have seen many places where it is in vogue to have outsiders have different alignments; the fallen angel or the non-evil demon or devil. But these races are the best of the good and worst of the bad or so they should be. So what chance to redeem, do you base it off of how much good or evil one radiates? Would an evil priest be much more difficult than an evil fighter? Is it alright to kill someone just because he or she radiates evil?
Redemption's a call for roleplaying-- one I'd feel free to make as tough and frustrating as you like. You're trying to change someone's whole world view-- and not in the easy way, towards selfishness.
I haven't dealt with it in a game yet-- to be honest, redemption's a long time effort and we usually play very short (gameworld time) periods. The possibility of redemption is usually foreshadowed-- if there's a hook, like the person you're trying to redeem was betrayed by another evil-- then it's worth an effort. Joe random Blackguard? Unlikely.
As far as killing someone because they radiate evil-- it depends on how you interpret it and how the PC is roleplayed. For me, Detect Evil detects a person's aura-- what they typically are, what's worn in as a pattern. If you allow it to catch transient thoughts, then there'll be a number of false positives (good people with revenge fantasies and the like) and you'd kill a lot of innocents.
In my homebrew I have a situation where one of the PCs has made a project of trying to redeem a half-drow assassin who was sent to infiltrate the party, then subsequently discovered and forced by circumstances to turn coat. It's pretty interesting, although the campaign is kind of on hold right now. I'm planning on testing the PC's commitment to this project with a little backsliding, as the assassin's organization catches up with her and threatens her into betraying the party. We'll see how it turns out.
It definitely requires a lot of roleplaying and a really detailed backstory for the PC/NPC, to explain why he/she turned to evil in the first place and provide hooks for how he/she might be redeemed. A conversion experience might play a role, but it should never be simply a matter of turning off the evil switch and "getting saved." Lots of life habits and thinking habits have to be changed, and you should encourage role playing to work out the struggles involved.