This is JUST for drug use.
Standard medical addiction. Many people who got on opiates do so thanks to legal prescriptions. Do these people all have MH disorders? Of course not, they're given incredibly addictive drugs that feel great and then just told to stop taking them.
Societies "education" on drugs in general. Most schools teach us that all drugs are bad and evil and you'll become an addict. Then kids try booze and pot and find out it's not actually bad at all. If they lied about those then what about all these other drugs I can try?
Young people are stupid and arrogant in the belief that nothing can harm them. Maybe you were a model citizen, but many young people do a lot of stupid things because they think nothing bad can happen. I know I said yes to way too many things in college, especially when I'd already been drinking. It's not that all college kids have MH issues, it's that kids brains are dumb. Problem is one dumb move with heroin can ruin a life.
Life can just be shitty. People all find ways to cope with their crappy lives. Some people choose to look any way they can to be happy. This could be a symptom of a MH issue or it could just be someone stuck working 60 hours a week in 2 shitty jobs and see no way out. Heroin is a cheap way to actually feel good for once... until you're in too deep.
Violence obviously has a different list, but yeah, go find any Reddit thread about substance abuse or violence. See how many top comments just talk about MH vs all the other risk factors that lead to these things.
And quit saying I'm jumping to conclusions. If you can't recognize that Reddit loves to pin bad things on MH and ignore other factors (besides gun control of course) then you need to get your head out of the sand.