Whether he is crying out for help, or has some other mental condition is immaterial.
Actually, that's very much important compared to someone with a "sound mind" who adopts similar behavior. It means they've fallen into a hole, either through vulnerability or carelessness, and they could genuinely be helped out of it with the right amount of effort. Someone who's not vulnerable though? Well, they still need help out of the hole, but chances are, they probably dug it, so getting them out would be a lot harder.
But violence? Ugh. I hate the idea... but it may come down to that, in the end.
But it didn't "come down to that, in the end" because you didn't know anything about the guy and didn't talk to him, or nothing. You saw someone wrapped in symbolism walking in broad daylight and your immediate reaction was anger. Which, that's okay. That would be my reaction first too. You're human, I'm human, we respond to stimulus.
The thing is though, like I said, there's a lot of helpful, legal ways to combat this kind of stuff. If it was me, first thing I'd do is check with my community leaders, such as the city council, the police, religious and school leaders, etc. and see if they're aware there's potentially a local chapter for a hate group. Then depending on what happens, I'd go from there. Me personally, I'd probably just give said leaders a heads up, and allow more experienced and knowledgeable people take over. There's tons of groups and resources out there for this kind of thing and tons of ways to illicit healthy, community responses. I know, because hate groups are kind of a thing where I'm at and the communities around here have done wonderful jobs handling situations as they pop up.
But bro. Again. Rash decisions made in anger almost always end poorly.