Everything can be redeemed, even devils and demons. But that almost never happens in the timespan a regular D&D game covers. Sure, you might have a few NPCs you check in with to see how their therapy and coping mechanisms are going, but the really big ones are going to be almost entirely in the hands of your DM.
The problem you face is how to meaningfully apply your attempts at redemption. That means putting them in touch with a willing mentor, or just telling them about resources that can help them escape their terrible circumstance.
Another problem you face is institutional violence. A goblin might be totally redeemable, but so programmed by his culture and kin that he tries to kill you even when you offer him mercy. Thats a tough one, because he's not evil by our metric. He doesn't like kicking puppies, but thinks he must because [reasons].
So pick your "battles" and accept that your party is going to kill most enemies. You only have screen time to redeem the big ones - a few major villains, and a few beloved NPCs at most. Know the local resources like shelters, mentors, etc to offer to NPCs. And make sure to share the spotlight.