So trying to toe the non-bias line; you can see from the drone footage that Rittenhouse is clearly trying to distance himself from Rosenbaum as quick as possible.
Rosenbaum gives chase (and throws something). Rittenhouse at this point stops turns and shoulders his weapon as a form of threat / intimidation (delete as appropriate to your bias) and de-escalate. This causes Rosenbaum to momentarily slow down. Rittenhouse then proceeds to turn back to the direction he was travelling and again tries to put distance between himself and his perceived assailant. Consider the "us" and "them" scenario he is faced with - he isn't part of the "mob" - he can only go where they aren't, in this instance it's the back of parking lot where unfortunately several vehicles are parked. Now I don't know about you, but trying to run with a rifle, at speed, and navigate through tightly parked trucks is not an easy task.
Unfortunately the result of that means Rosenbaum was easily able to close the distance, at which point Kyle has ended up stopping his perceived threat through the discharge of his firearm, four times, at very close quarters.
In my opinion he has pretty much exhausted all options available to him. His sole drove at that point was to put distance between himself and a very aggressive man that had prior threatened to kill him. He's attempted de-escalation to no effect and when, essentially, cornered has defended himself. Did he panic? Probably. Was it excessive? You could definitely argue it was in hindsight. But in the moment, in a hostile environment, with your escape routes blocked and someone who wants to harm you within arm's distance, I can perfectly understand why he shot.