A Logical analysis of this argument in the Rittenhouse trial.

"Mr. Rittenhouse, why didn't you let the person trying to kill you kill you before you decided to defend yourself."

Seems like a presupposition that the person he killed was going to kill him. After all, that assumption is unknowable and we cant ask the dead their intention. 

This also seems to ignore the context. Context being that Rittenhouse was Blue Lives Matter who went to a Black Lives Matter protest with an AR-15 that was not his property, and he had no property or business there he was defending. Seems he went as a provocateur. And shots were fired, and the crowd identified Rittenhouse as an active shooter so they were trying to stop him from shooting anyone else. 

It also seems like an appeal to smart a** fallacy, kind of like the appeal to ridicule because of course we all know he cant shoot someone if he is dead first.

Anything else you can add that I am not seeing?

"Mr. Rittenhouse, why didn't you let the person trying to kill you kill you before you decided to defend yourself." 

This isn't an argument, but we can make it one. If we did, I would imagine this would be a strawman fallacy as I doubt anyone ever suggested this. But defense lawyers constantly commit fallacies—that is part of their job. Reason and truth is not their goal—it is defending their client, which often involves manipulating the jury with emotion and rhetoric.