Many times they don't and there is no history of mental issues.
It does exactly that because not all mental illnesses are the same but it gives the public at large the illusion that those who are more likely to be victims of violence are more likely to assailants.
Hitler, Pol Pot, Stalin, Andrew Jackson, they all more or less woke up one day and decided millions must die.
Yes, healthy people kill other people. Try actually studying real crime, including "regular" murders, mass shootings and serial killers. The serial killers are the only ones with a consistent history of profound mentel illness. Everyboyldy else more or less is just human amd accidents happen, emotions flair, people get pushed over the edge and dire situations happen.
Yes, some shooters do have a history of mental illness, but lets not lump autism and adhd in with the criminally insane.