Between then and when he shot 3 people, he had armed himself, taken up a position at (IIRC) a used car lot, and in his interview, described himself as a "medic" for the vigilante group he was with.From what I’ve read Kyle Rittenhouse was there because he
worked that day as a community lifeguard in Kenosha. After work, he wanted to help clean up some of the damage, so he and a friend went to the local public high school to remove graffiti by rioters. It does not appear that he went there to insert himself into the situation as a vigilante.
... so it seems at some point, he did decide to insert himself into the situation as a vigilante.