Mental illness. Pure and simple.
Doesn't have to be mental illness as such. Could be a personality disorder, which is just basically saying the he's not ill in any treatable way, rather just 'born bad'.
Same way as you don't classify people born deaf as 'physically ill', someone with a psychopathic type personality (no empathy etc.) isn't usually considered 'mentally ill'.
This is not to say that he should be let off because it's 'not his fault'. It's perfectly possible for many people to control their personality disorder intellectually (e.g. I want to do X, I know it won't make me feel guilty, but I don't because intellectually I know X is a bad thing for society or whatever).