That's to say, every society has its discontents who take their anger and frustration and discharge it on innocent civilians. Some societies manage their mentally ill people better than others. We do a pretty pitiful job at it.
I think this is one of cases where a broad label like "mental illness" obscures more than it clarifies. There are some subgroups of people with mental illness (schizophrenia, bipolar, antisocial personality disorder) who are more likely to commit violence than others, especially when combined with substance abuse. But of course that fact doesn't generalize to all people with mental illness.
EDIT: I used ChatGPT 'thinking mode' to get that number for USA. Where mass shooting >= 4 people injured.
> EDIT: I used ChatGPT