It could be a Manson-like attempt to start Helter Skelter. It could be people who think the BLM movement isn't militant enough. It could be terrorists wanting to kill people for maximum carnage who had no connection to either side. The reaction to this will largely be based on that.
I'm a Dallas native, and it's a really hard thing to wrap my head around it happening in my city. The only thing I hope it results in right now is proving this dumb idea that open carry somehow makes things safer is idiocy.
(Oh, and the person pictured above actually isn't a suspect - he was videotaped, with his weapon, standing on the street during the shootings. He turned the weapon over to officers immediately in the aftermath, but I'm guessing the communication from those didn't make it up the chain of command in the chaos. He's turned himself in as a protective measure, but it's pretty clear he wasn't involved.)