In general, there seems to be a tendency to overestimate the intelligence of people who do evil things. This is known as the Hannibal Lecter myth named after the 150+ IQ evil psychopath from the Silence of the Lambs movies.

I think the reason the Hannibal Lecter myth is so popular is that the reason intelligence evolved in the first place was survival (the mental ability to adapt situations to our advantage), and prior to evolution of higher morality, primates were largely motivated to use our intelligence only for selfish purposes (though all conscious behavior is selfish on some level), so sadly, on an instinctive level, intelligence used for evil goals is the intelligence many people recognize most easily.

Sadly, many people secretly admire evil, but they can’t come out and say it, so instead they just say “he was an evil genius, a terrible human being, but brilliant. Just brilliant!!!” This is a safe way of praising an evil-doer, while making it look like you are condemning them. It also makes people feel sophisticated, because they are expressing the nuanced understanding that people are multi-dimensional (positive traits like intelligence can coexist with negative traits like evil).

Another reason for associating evil with brains is that many mentally retarded people are the opposite of evil. They come across as the most loving, trusting, friendly, giving people you will ever meet. So if the most good people are often mentally retarded, it’s only natural to think the most evil people are geniuses.

Evil geniuses and saintly retardates notwithstanding, the reality is that evil people are probably less intelligent on average, because the same brain defects that impair one’s compassion for others, also likely impair one’s cognitive function, since the functioning of one brain system is likely correlated with others.