And he is not alone.
There is a meme out there that gun laws do nothing because well-armed criminals will ignore them, so they are useless. I contend that most of the shootings that put the general public in danger are not from those well-armed criminals - they are from law abiding citizens without a criminal record who simply snap and grab their AR-15 before they run out the door to get their revenge or make their statement.
To demonstrate this I looked at the last ten mass murders, taken from Wikipedia's list of mass shootings in the US.
Yakima shooting - the shooter, Jarid Haddock, had no criminal record - just a semiauto handgun and a lot of ammo.
Half Moon Bay shooting - the shooter, Chunli Zhao, was a disgruntled worker with no criminal record.
Monterey Park shooting - the perpetrator, Huu Can Tran, had one previous arrest for illegal possession of a firearm with no other arrests. No conviction. There was no apparent reason for the shooting but he did appear mentally disturbed beforehand.
Goshen shooting - two suspects arrested. No clear motive yet but it may be related to gang activity.
Enoch shooting - a man killed his family and himself. No criminal record although he had been investigated for domestic abuse allegations, and his wife was seeking a divorce.
Chesapeake WalMart shooting - a WalMart employee with no criminal record shot several people at WalMart then killed himself. He left a note with allusions to Satan and being mocked at work.
Colorado Springs - Anderson Lee Aldrich shot up a nightclub. He had one arrest previously for "taking his grandparents hostage" with a weapon and threatening to blow up their house - but was not convicted of anything.
University of Virginia shooting - Christopher Darnell Jones Jr shot and killed several other students. He had no criminal record, although he had been under investigation for possibly having a gun at school.
St Louis shooting - Orlando Harris shot and killed three schoolkids. No criminal record. He was being treated for mental illness. He tried to buy a gun from a dealer but failed the FBI background check. He then bought one from a private individual. His mother was horrified when she heard this and called the police. The police came, but said that he had broken no laws, so they could not take the AR-15 from him.
https://en.wikipedia.org/wiki/List_of_mass_shootings_in_the_United_States
Of those ten, one MAY have been pulled off via one of those hardened criminals that gun supporters always reference. The rest were people with no criminal records who just snapped. It is also worthwhile to note that in at least half of the cases, there was a clear sign (former misuse of a weapon, mental illness, or threats of violence) that this person might be a danger if armed.