He was under age to have a firearm at all, except at a range, hunting, or in the military.
He knew carrying a rifle would deliberately be provocative.
In effect, he was trying to intimidate the entire protest.
So he had violated his "self defense" rights.
He deliberately caused the situation to happen, and it was totally predictable that people would try to disarm him.
{...
Rittenhouse fired an AR-15-style weapon eight times in all during the unrest: four shots at Rosenbaum, who was unarmed; two shots at an unarmed unknown individual who kicked Rittenhouse; one fatal shot at Anthony Huber, who hit Rittenhouse with a skateboard; and one shot at Gaige Grosskreutz, who was holding a gun, according to prosecutors.
...}
Clearly there was no deadly threat that warranted any shots by Rittenhouse, and obviously 8 shots in a crowd like that is incredibly irresponsible.
It is obvious in the video that Grosskreutz had a MedTech uniform on, and was pointing his pistol up in the air when shot.
Rittenhouse's bullet went perpendicular to Grosskreutz's forearm, which can only happen when pointed up.