Which is exactly why nothing evil should EVER be killed by good characters in D&D.
What's wrong with killing things that aren't evil?
Self-defense and defense of others don't just fly out the window because it's not evil. If my PC's were attacked by a crusading angel of goodness and light who attempted to burn them to ash for some vendetta the creature had, they wouldn't shift to evil for killing it. If my PC's were attacked by a troop of evil orphans, clearing out their evil orphanage wouldn't be evil.
"Being evil" isn't a reason to kill something any more than "being an orc" or "being over the age of 50" or "being a person with green eyes." Similarly, "not being evil" isn't a reason to stay your hand, any more than "not being a person with green eyes" isn't a reason to spare their life.
It's about what they do, not what they are.
If you're playing the Dungeons and Dragons where the goal of your PC is to fastidiously avoid the accumulation of wealth and power in your pursuit to redeem the souls of evil creatures through... I don't know, tough love or something-- then you have deviated just a teensy bit from the core experience of seeking out evil creatures, slaying them righteously, and accumulating wealth and power so that you can slay ever more powerful evil creatures.
I think you misapprehend the core experience. The core experience isn't to seek out evil creatures, slay them, and repeat on an escalating scale. The experience is to stop evil, then stop greater evil.
The difference is that you're not seeking out evil creatures to slay, you're just stoping evil acts from being done (by evil creatures, by good creatures, by neutral creatures). If the neutral thief is murdering neutral citizens in the neutral street, you don't ignore him and go fight the evil goblin five kingdoms away just because he's not evil and the goblin is.