A situational emotional crisis without a prior history of psychological problems is not ‘mental illness’.
Mental illness is chronic. Stuff like recurrent depression. Recurrent mania. Vacillating between both. Both at the same time. Aural hallucinations and voices in your head that tell you you’re no good or that you should kill yourself. Social anxiety so bad you never leave the house. Depression so bad you never leave the house. Both. Bursts of panic that come out of nowhere.
People with mental illness are, on the whole, far more likely to kill themselves than the general population and actually slightly less likely to be homicidal.
People without such history are not immune to these states of mind. It’s just that it takes a lot of psychological and/or physical stress to induce them. In their everyday lives, these people would pass a psychological intake just fine, which is why gun reform targeting the ‘mentally ill’ is total, utter BS.