If we had to pick only one demographic descriptor to predict who is most likely to kill another human being that demographic descriptor, in all countries would be sex. Men are by far the majority of all killers, though of course, the majority of all men are not killers at all. But a minority of men does almost all the killing.
So it would seem very important to try to understand why this is the case. Is it biology, bias in how boys and girls are brought up with more entitlement allowed for boys (and so more later rage at not getting something they assume was theirs by right)? Or is it a complicated soup of all these things and more?
Gun availability matters, for the same reason that using a lawn mower rather than nail scissors to cut grasses down in a lawn matters.
Some have argued that this means women and men are equally homicidal in nature, but because men are physically stronger, on average, they end up completing more killings. IF this is the case, then countries with easy access to guns should find the female and male perpetrator rates for homicides move towards equality.
This is not happening. The question, then, would include looking at upbringing, the expectations about men's roles and the entitlement levels each sex may be allowed to have. And also at biology, including testosterone and its effects. And how these, and other factors probably all interact in various ways. For instance, testosterone clearly doesn't cause most men to turn into killers (in wars they are essentially forced to do that), so even if the explanation is biology-based, there's more to it.
I have noticed that after each new mass slaughter event in the US there's lots of debate about mental health and access to guns, but it's extremely rare to see any attention aimed at the best single predictor of who is going to kill, i.e., certain sub-groups of men and boys. It is not on the table, at all.
Cases where "the man bites the dog", i.e., where it is a woman who commits mass murders allow some of that to enter into the debate, though even in those cases there's more focus not on how rare such cases are but on how we are all going to hell in a hand-basket if even females now kill!
The gun debate in the US might seem more familiar to you if you think of the pro-gun lobby as equally powerful to something like Stonewall, and if you think of many gun owners thinking, just like some trans activists, that the alternative to what they want would be their annihilation.
The pro-gun side in the US don't believe that the criminals will give up their guns, and so they regard any restrictions as leaving them the hapless victims of the criminals. But these arguments and beliefs are very much dependent on which US state we talk about, too, i.e., in many places all are for guns and in many places none are for guns.
So to return to the actual topic here, the possible preventable causes for this ghastly butchering (easy access to guns, untreated severe mental disorders and possible effects of cross-sex testosterone) all are important to research.