So, the sequence seems to have been that he killed two at a/his residence, his mother and stepbrother. I had speculated earlier that they were his parents, so I was close (not exactly a brilliant deduction

).
Then he went to the school he had attended - graduated? dropped out? He was 18YO - where he shot a bunch of students/staff, 7 of whom have died. Then he shot himself.
Speculating based on some similar shooters' motives, it looks like he hated his life, killed those he could of his family, and then went to the school to make a splash, spread his pain, and give himself some twisted sort of "significance".
While I think his being trans was a relevant part of his mentality, I'm more interested in the pain he brought on the families whose children and/or adults he killed and their friends. It's a small town, so probably every resident has some sort of loss.
Apparently, he had done hunting, so nobody smuggled a gun to him.
Did a
DU-Faithful really sort of sideways-allude to the
OhBummer! MalAdministration's
Operation Fast and Furious?