I mean, there's probably something to that. Unless one wants to try and make some ridiculous argument that specifically American men are genetically predisposed to shooting children, there is definitely a fault of culture at play. Women and men are subject to different forces and pressure in the culture we have, and someone being subjected to the forces aimed at men, for whatever reason, may very well reach nearly male-exclusive conclusions.
I actually agree with that. Gender, as performance and assigned values, is quite independant from sex, which is its own complex thingy, and all combinations occure. The only trap is mono-causality, and it's very seductive for its great offering in terms of "oooh i understand everything" feeling for minimal data input, study and thought process (we function on brain economy above all).
Murders by women do happen, just like domestic violence by women, rapes by women, etc. They're "merely" largely more rare than male violence. But they can be driven by many things, from the generality of our masculinist values (our fictions glorify violence, and tend to attach it to manhood but without any explicit exclusivity) to individual freedom towards gendered values (a lot of people reject the dominant dichotomy and embrace values that we arbitrarily gender as "not theirs") to completely transversal reasons (there are so many feelings and pulsions that are universal).
Trying to find one causal, gendered, cause to a mass killing is absurd, and is just a projection of prejudices - like learning that the killer used to listen to rock&roll and suddenly deciding this is the determining factor, while you didn't consider the musical habits of other killers as relevant (it's the "how it works" xkcd strip). But at a statistical level, when assessing the overwhelming majority of male mass killing and the role of gender socialization, it's not absurd to consider that a person treated as a boy since birth could have been socialized accordingly to traditionally male values and internalized many of them.
Let's just also keep in mind that 1) a lot of boys socialized through these values do not kill, 2) a few girls embrace these values despite mainstream cultural gender biases. Transgenderism is neither necessary nor sufficient for a girl to align with statistically male categories. It has a weak, hypothetical explanatory power. And also, absolutely nothing to do with biological determinism.