How to make a mass shooter.
1. Modify the government's crazy person registry to exclude radical leftists.
2. Take an ordinary radical leftist.
3. Make him/her psychotic, with either LSD, Meth or whatever until they lose their minds. OR find a mentally ill radical leftist (it's not too hard, they're all mentally ill).
3. (optional) Tell him/her that they are oppressed.
4. Make any semi-automatic handgun or shotgun available to him/her to take.
5. Wait until Democrat politicians need to send another an anti-gun message to the country.
The vast majority of mass shooters are leftists. Leftists hate that this is true because TV teaches them that is it right wing conservatives. TV is wrong but you cannot convince the left of that fact.
All other mass shooters (and this is a small percentage) are truly psychotic and have no political affiliation whatsoever. They could not answer what a liberal or conservative stands for.
After that it is Muslims. There are about 25% of Islam who follow all of the Quran and are waging the same holy war (jihad) that Allah told Mohammad to wage. They're goal is convert or kill, with 1 exception. When it comes to Jews and Christians, you can kill them or you can tax them to death. The option of killing or taxing belongs to the Muslim who confronts the Jew or Christian. All other unbelievers, you should just kill them, even if you don't want to kill them, Allah commands it.