There are more male killers because they are more physically capable.
Or perhaps because they are less physically capable of killing, and evolved ways of getting what they want through controlling men, and subterfuge (and things like poisoning, sure) then whatever pathology leads to serial killing is less expressed in the female mind, and that's why there's fewer of them. That's a bit different than the implication you're drawing, that if they could just get away with it there'd be an equal number or more female serial killers.
Or perhaps because they are less physically capable of killing, and evolved ways of getting what they want through controlling men, and subterfuge (and things like poisoning, sure) then whatever pathology leads to serial killing is less expressed in the female mind, and that's why there's fewer of them. That's a bit different than the implication you're drawing, that if they could just get away with it there'd be an equal number or more female serial killers.