Which any reasonable person in that situation would have believed, with the information available to them at the time.

Had Rittenhouse dropped his gun, he would not have been an imminent threat to the crowd. But he did not. For all anybody on the scene knew, he would start shooting again, as soon as he had a chance to.

Disarming him was one way to remove that imminent threat of death and great bodily harm, which Huber tried to do. Killing him would have been another, which Grosskreutz considered, but did not try to do.