Because the only communists famous enough for you to think of them (as famous communists) are famous for being in charge of big important countries, and people in charge of big important countries tend to be male and often kill a lot of people.

This certainly explains the maleness, and admittedly I don't know what the gender ratio of communist theorists is like. Nevertheless, communist countries have been responsible for a disproportionate number of massacres.

To be fair, I seem to remember that women are more left-wing on average, but the effect size is pretty small. Equally, 'angry young men' are often very socialist because they want to 'fight the power'.

And yes, I'm clearly not saying all communists are killers, that would be crazy. And if you are communist because you live in the Soviet bloc, then that's due to completely different causal factors.