Intelligence IS hard to define and different people have various types of intelligence - but there is certainly a degree of intelligence that is heritable - and thus based upon genetics.
So why would you presume that if there WAS a “murderer gene” that it would lead to disproportionate reproductive success?
The intellect, like the will, is essentially spiritual, yet associated with the brain. If the intellect was purely material, it would reduce to a machine, capable of error, placing all thought in fundamental doubt. But since we can know some things with certainty, this premise must be false.
My point regarding the “murder gene” was meant to highlight the problem of reducing the mind and will to material phenomena.