Pretty much the whole point of this fucking stupid ass story called Monster I've been quietly reading out of sheer boredom. There are flawed people in this tale, including murderers among others, but it tries to demonstrate evil is subjective and the closest we'll get to it is if someone twisted simply refuses to change (like the titular villain). Since then I've been talking to a more optimistic friend who claims redemption is possible for anyone, but we live in a world where this isn't a thing since we just opt to throw our world's bad guys into prisons to rot or even kill them with no therapy or anything.
I am, however, of the belief evil people cannot change and so trying to help them is meaningless.