Trans is a red herring here.
Why the feck are so many Americans going on shooting sprees? So many of these incidents end with the shooter being killed, either by police or shooting themselves, that we don't get any useful information. Admittedly, we've not got much information from the headcases thst carried out the attacks Southport or Luton - but at some point, we might.
The real problem seems to me to be the inadequacy of adolescent mental health services, and the difficulty in getting unwell children to actually engage. Ive been through it with my own child - you can't exactly drag them into CAMHS by the hair, and even if you could, you can't force them to interact with anyone. The more ill they are, the less they can engage. I can't imagine it's any different in America.