Uh, the First Amendment doesn't protect rioting and looting. There is a clear difference between that and protesting.
I agree with your next point though, Rittenhouse should never have been playing "hero" and nor should anybody else. Because the police should have stepped in during the previous two days of unrest. But they didn't. The governor effectively stood down as Kenosha burned.
If I lived in downtown Kenosha or had a business there, I'd probably have been glad for any protection I could get.
Without the looters, Rittenhouse would never have been there. If the police had taken decisive action, Rittenhouse would never have been there. If there were sensible gun laws, Rittenhouse would have been liable for jail time the moment he strapped a semi-automatic weapon to his chest.
I'm not making excuses for him. He was almost unimaginably irresponsible, minor or not. But the circumstances that allowed for this tragedy run much deeper than the stupid decisions of one child.