I think the Dayton shooter fit more of what you're talking about. He seemed to be clearly a problem. Probably the Parkland shooter as well. The El Paso guy wasn't. No one had a clue. He was on no one's radar. Yoga shooter, the Toronto dude who used a vehicle, Eliot Roger in California, and a host of many others. You can bring back institutionalization, and clearly some people need that. But that's not going to stop mass shootings.
No single thing will fix this problem. No single thing is causing it. That doesn't mean that you stop trying to address it.