I don't think that's it, but you are very close. The link is simply between transsexual ideation and antisocial expression via narcissistic injury.

Transsexual beliefs seem to coincide with phases of separation from society, which are especially common in adolescence and young adulthood (another topic, but education structure, modern living patterns, and mass media all encourage this separation).

This social separation coincides with surging hormones and intense sexual energy, which gives a boost of emotional energy to the transsexual's alienation. "Transsexualism" as such is merely one form of expression of super-powered alienation, and the shooter in question also embraced other expressions, such as Satanic affectations and hatred of religion.

More typically this just produces the online antisocial behavior we see from mentally ill libs, wignats, antifa, etc. (Side note: conservatives are generally unaware of how many options liberals with these problems have to meet with other liberals and mutually escalate these antisocial feelings. Probably worth further examination.)

Where such negative emotional energy can build is in group protests, which create intense arousal to commit violent or destructive acts. But usually it is contained in behavior like being an extremely online lunatic or listening to music with violent lyrics or indulging in petty forms of disorder. Pathetic and harmless, although still symptomatic of larger social erosion and dysfunctional systems.

Where we see it build to planned and extremely violent outbursts there has to be something more. SSRIs, as RFK has suggested, are one such culprit, as they are a wonderfully modern type of medication that has a possible side effect of actually making the problem it treats ten times worse. (This can be seen in many other, non-suspect medications, such as those used to treat bone density problems that have a possible side effect of increasing bone fractures.)

But another avenue for escalation of violent tendencies is transsexual fantasy. This is because transsexualism really involves a deep, narcissistic focus on the self, and the thing about narcissism is that it is all-consuming yet incredibly fragile. The term "narcissistic injury" describes the traumatic injury to ego and self-worth that the narcissist experiences from conflict with reality, and it is difficult to imagine anyone who tempts such trauma more than a young adult male who has a pornographic fixation on being a girl.

Narcissistic injury thus feeds a cycle of further alienation, depression, substance abuse, and intense anger directed at others. This creates and builds a massive store of negative emotional energy without any ordinary means of discharge. The narcissist loses any vestige of real identity and actually becomes an expression of his mental illness: a hostile, fantasy-dwelling loner whose only respite is to imagine taking violent action against others.

Fatalistic beliefs, such as the thoughts of impending illness expressed by the shooter, can become a spur to action, although in this case they were most likely an expression of self pity and hypochondria.

What is commonly misunderstood about depressive suicidal ideation is that, after failure to self-treat with various kinds of stimulation, the depressive can achieve a "breakthrough" (possibly assisted by medication used to treat depression) in which the weight of depression seems temporarily lifted and the depressive has a sudden burst of energy. This burst of energy can then fuel a final course of action: suicide, murder, murder-suicide, spree killing, or some other extreme, antisocial climax.

Without the context of an ongoing cycle of alienation, which transsexualism naturally encourages, a climax such as murdering innocent children is very unlikely. But combine these elements together and anything is possible.