You might be curious to find that in many animal species studied, from pack animals down to ants, there is always a large percentage that contribute nothing and are a net-drain on the larger life-structure or colony. Humans and all other forms of life seem to share this commonality.
Yeah, pick any two humans and put them in a similar situation, and I truly believe that you'll see similar empathy 99.9% of time time. But that fucking 0.1%, they're ruthless and they're rewarded handsomely for that behavior.
It's more just that they aren't punished for it. They don't have the empathy to give a shit, and thus will do things regular people won't. If society doesn't punish them for being a piece of shit, then there's no downside to being a piece of shit for them, only upsides from taking advantage of situations others won't.
A lot harder to punish when you can start making the laws as well. Society won’t just reward them sometimes they will let them write what everyone else should do as well.
This is why you have to introduce the concepts of mimics or demons that have access to change shape. Otherwise the party always frees the chained up maiden in the dungeon without asking any questions. Alternatively, if there is a rogue, you don't have to worry. They'll try their best to convince the others that they'll get xp for stabbing the prisoner.