I don't support vigilantism, as a general rule, if that was unclear. But I'm also not a Pollyanna who thinks it could never possibly be warranted, under any circumstances. Life is complicated. Vigilantism should be an absolute last resort, and even if justified, isn't the job of children, unless they absolutely have to defend themselves because there's no one else to do it. So, Rittenhouse shouldn't have been there, period -- I don't disagree with that at all.
But I do think that when the adults abnegate their responsibilities, undesirable consequences are likely. Having the police stand down in response to the 2020 riots was exceedingly reckless and naive. We got exactly the results one would expect, and honestly, I'm surprised we didn't see worse.
When you remove civil society's protections from people (explicitly not protecting them, their homes, or their livelihoods), it's naive to expect they won't revert to pre-civilized recourse. After all, isn't that what many of the justifications of rioting essentially were too? Black people weren't being protected by society, so it was all an understandable expression of grievance? Unsurprisingly, it's also the justification of the vigilantism.
The world works how it works -- no amount of scandalized pearl-clutching changes that, on either side. That's precisely *why* enforcing the rules of civilized society, as evenly and justly as possible, is so important.