Whether a person feels pleasure, pain or indifference at the misfortunes they render onto others seems to be largely reliant on the narratives they operate under which justify, downplay or discourage that behaviour. Familiarity with a task also tends to mitigate the negative emotions associated with it. When thinking about psychopaths, I tend to wonder at the road that lead them there. If they were educated to that point, I don't think the label of psychopathy helps us to understand what's going on.
I personally don't think that psychopathy is a mental illness in the same way that epilepsy is a mental illness, but that it's probably just a pathologization of certain behaviours that society deems as undesirable (often for good reason).