Speaking as someone who used to worry about people so much, even those large degrees from my circle, that I'd drag myself down to very depressive episodes, I can see how some amount of psychopathy would be selected evolutionarily. Besides that, any work where you're dealing with people in pain such as a soldier, police, medics, etc, it's not often beneficial to be squeamish to others' pain, especially as a medical professional since the pain (negativity) you induce will often have positive effects. Perhaps psychopathy is just an extremely selective form of this.