>or who stole the weapons from someone else,
This rings alarm-bells with me, let's have a look:
https://en.wikipedia.org/wiki/List_of_rampage_killers_%28Ame... says (sorted by year)
First one:
>In September 2012, Rodger visited a shooting range to train himself in firing handguns.[12] In November 2012, he purchased his first handgun, a Glock 34 pistol, in Goleta, after doing research on handguns and judging the Glock 34 to be "an efficient and highly accurate weapon", as documented in his manifesto.[16]
https://en.wikipedia.org/wiki/2014_Isla_Vista_killings#Prepa...
Second one:
>The Glock 9mm pistol used in the shooting was legally purchased by Vargas in 2010 from a local gun shop, and he had a concealed carry permit for it.[8][18][3]
https://en.wikipedia.org/wiki/2013_Hialeah_shooting#Prior_to...
Third one:
>On May 22, 2012, Holmes purchased a Glock 22 pistol at a Gander Mountain shop in Aurora. Six days later, on May 28, he bought a Remington 870 Express Tactical shotgun at a Bass Pro Shops in Denver.[64]
https://en.wikipedia.org/wiki/James_Eagan_Holmes#Events_lead...
Fourth one:
> Following an incident later in 2007 involving his stepfather, a restraining order was filed against him, barring him from possessing firearms. The order lasted a year and had expired at the time of the shooting.
(Seems to imply the guns were his? Can't see in article)
https://en.wikipedia.org/wiki/2013_Hialeah_shooting#Prior_to...
Fifth one:
>Loughner allegedly purchased the 9mm Glock pistol used in the shooting from a Sportsman's Warehouse in Tucson on November 30, 2010.[28]
https://en.wikipedia.org/wiki/Jared_Lee_Loughner
Sixth one:
>He got out, shot, and wounded a woman on a motorcycle with a Norinco Mak 90 semiautomatic rifle.
(doesn't say who owned it)
https://en.wikipedia.org/wiki/2011_IHOP_shooting
Seventh one:
> Items found on Wong's body included a hunting knife, in the waistband of his pants;[13] a bag of ammunition, which was tied around his neck;[27] and two semi-automatic pistols (a .45-caliber Beretta Px4 Storm and a 9mm Beretta 92FS Vertec Inox, matching the serial numbers on his New York State pistol license)
https://en.wikipedia.org/wiki/Binghamton_shootings
So of these 7 rampage shootings, 5 were done by people who used their own gun, one is implied (possibly not?).
Furthermore, some of these had past known mental health issues: Elliot Rodger, Scott Evans Dekraai had a restraining order filed against him, Jared Lee Loughner was told to get his mental health evaluated if he wanted to return to university (later diagnosed as paranoid schizophrenic), Eduardo Sencion was diagnosed with paranoid schizophrenia a few decades before the shooting, James Eagan Holmes was seeing several psychologists of which one tried to warn his campus police, the others have unknown motives.