Mental health is a huge, but its fuzzy.
A lot of people give you signals before they do bad things, some don't. In the case of the domestic violence murders I've been exposed to, everything was normal from the outside until the wife either announced she was leaving or admitted to having an affair. In all three, the husband snapped killed her, in one case the kids, and then himself in a matter of hours.
I have a close family member who is showing stages of early onset dementia. That side of the family is very pro gun. Their mother loses her gun quite often and its found in the couch, under the car seat, or some other random place. Family with lots of young children laugh about it, but no one wants to hurt her feelings by telling her she doesn't need a handgun.
I have multiple friends (former friends) who in the past few years have started building arsenals and prepping. They have become overtly racist (replacement theory/ready to protect retail stores from non-existent BLM riots in our small town), conspiracy driven (qanon/religious end times/anti-establishment), and quick to anger. Some times they are same people who would help anyone and sometimes anyone who is not like me is out to get me. I don't know if there's any mental issue other than being led to believe a bunch of insane stuff. I don't think any of them would necessarily initiate violence. I do think that some could perceive a situation in an outlandish way and do something monumentally stupid and violent.