I'm thinking along the same lines that Rittenhouse heard the shot, turned around and it was at that point Rosenwhatever had caught up to him and attempted to take his rifle away after which time Rittenhouse shot him in the attempt. It appears quite clear to me that those dirtbags who were after Rittenhouse were the aggressors. Self defense all the way.
It's really not difficult at all:
1. There is no (known) evidence that Rittenhouse did anything otherwise unlawful or did anything to initiate/instigate force used against him
2. Rittenhouse was fleeing/running while being chased by an angry mob
3. One member of the mob chasing him threw something at him, others were shouting, and another fired a gun while chasing him.
4. The member of the mob who threw something at Rittenhouse while chasing him eventually caught up with him and tried to disarm him.
All of these together - and even some of them individually - clearly justify his use of deadly force in self-defense. And these are all articulated by the affiant as PC in the charging document.
The defense barely needs to do more than examine the affiant on the stand. The State made the self-defense case for him.