It was not a protest, it was a riot. He brought the rifle to protect property.
He was alone, because the police did not let him rejoin his group.
Rittenhouse was obviously the good guy, which is why you bad guys are hostile to him.
The police had any potential for riot under control, and Rittenhouse was not a local with any property to protect.
Nor was he doing anything to protect property, which would have meant staying with others to form defensive perimeters.
He was instead going up to individuals and challenging them.
The police and counter demonstrators most certainly did let people join them, and Rittenhouse immediately did run to the police after he started shooting.
It was not Rittenhouse's city, property, or interests.
He did not even know which side was in the right.
Nor is a rifle the right way to express political beliefs.
What if all the demonstrators or rioters had been carrying rifles?
The police would have had to shoot them.
Anyone carrying a rifle had to be shot.
No one can be allowed to carry a rifle in a demonstration or riot, (except the individual property owners, on their own property).