Self-defense is a response to a threat from someone else, "putting yourself" into a situation doesn't change that. If that were true, we'd be free to blame victims of other crimes (e.g. cyclists and pedestrians hit by cars) for putting themselves into dangerous situations. But that's absolutely not the case, it's not my fault if a car hits me while I'm legally riding/walking on the side of the road, nor is it my fault that someone attacks me because I'm holding a firearm.

That said, Rittenhouse was a minor and AFAICT not legally allowed to possess a firearm in that situation. That is the problem here, and anyone who enabled him to bring a firearm to that situation should be held at least partially accountable. But his actions in the moment were self-defense.