I think you are looking through the situation through the lens of emotion. "I felt unsafe because I heard a gunshot" is most likely not going to fly for stopping, and turning and shooting an unarmed person, running at you or not, especially given that Wisconsin doesn't have a 'stand your ground' clause.
The question before the jury, is going to be was he legally in a position where deadly force was justified in self-defense, if that is where his attorney goes with his defense, and that is going to be predicated on whether or not he was justifiably in fear of his life or great bodily harm from Rosenbaum. Rosenbaum was unnarmed, and didn't fire the shot that Rittenhouse heard, and that is going to come into play in whether or not stopping after the gunshot instead of continuing to retreat, negates self-defense or not. It's definitely not this clear cut case of self-defense from a legal standpoint.