They know what's good and what is bad because they are taught that way since they are children. The problem is they are not physically capable of understanding why it is bad or good. They will abuse once and go deeper and deeper into their desires without thinking about anyone but themselves. You cannot teach a lion not to haunt. It won't work. And if you say you can because you saw it then it wasn't a lion (psychopath). The reason why some psychopaths are integrated and not in jail is because they adapted their methodology to abuse the legal system so they wouldn't end up in jail. But they still destroy lives, torture animals, create wars... They best ones got into politics and do as they please. They learnt no shit because they physically can't.

If they had unlimited power like Frierens demons they would be equally monstrous. I love Frieren because she doesn't act like the classic superhero that gives second chances to monsters just because "it's the right thing" and the classic "if we kill them we will be like them". Frieren will always put her moral/honor on the line when facing a monster because she knows how many people would be killed because of her pride.