"he didn't mean it" is not a legal or moral defense.
If someone's officially stated reasons damn them then there is no reason not to take them at their word. That doesn't mean everyone has to be taken at their word, but there are more argumentative steps involved in going from "You say X when it was actually Y because..." than in saying "You claim X, so let's assume X is true...."
I'm not going to argue with you about his "true" reason for being there when Rittenhouse himself says otherwise. You say he went to Yap about a political issue, I say he went there to have an excuse to shoot someone. So let's meet in the middle at Rittenhouse's stated reason. His official reason was "to defend property" which he stated in court. And that was deemed perfectly fine by the legal system and the people parading him around to give speeches.
As far as the the legal system and the people supporting him are concerned, Rittenhouse went there to defend property, ended up taking lives as a result, and that is perfectly fine.