All I can say is that this honestly feels like a major case of denial on your son's part. Basically, he was at peace with a certain aspect of himself until his mother screwed him up and convinced him to feel shame about his transgender leanings. I'm sure it took a lot on his part to bury that side of himself. Nearly anybody here can attest just how deeply we can sometimes bury these feelings in our denial, and how painfully they can hit when they again come to the surface.
Perhaps it is time for him to go back to therapy. It might not be a horrible idea to have some kind of joint or family sessions, as you don't want him to feel singled-out or isolated. I'm not sure if this advice will help, but it's all that comes to mind at the moment.
I understand that this must be very difficult for you. Not only are you being rejected by your son, but you are potentially watching him reject him or- let's face it- her self.
Even if he is not willing to talk about trans issues with a therapist, there is something deeply disturbing about the fascination with killing, Hitler, and various other dark subjects of that ilk. These interests are very indicative of a feeling of depression and perhaps even powerlessness. It could be trans issues, or it could be something else. Hopefully, whatever is wrong can be resolved in a therapist's office.
The tricky part with boys his age is getting them to believe that you are trying to help, rather than hurt. Keep reminding him that you love him, and it might not be a bad idea to share with him that he scares you.
I'm sorry I can't be of more help. Please, keep us updated and let us know if there is anything we can do to help, even if it just means providing a friendly ear.
Good luck, and be well.
Sincerely,
Tegan