Since Rittenhouse was quite apparently running away, and Grossenkreutz was pursuing him, in no way can a reasonable conclude that Rittenhouse was an imminent threat to Grossenkreutz — if he didn’t pursue him, he’d be completely safe.
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.
Not waiting around for that determination seems to have worked out pretty well for Rittenhouse. He's both not dead, and a free man.
Rittenhouse felt threatened, but for some reason, didn't wait for a court to make a determination. The people he shot also felt threatened. And for good reason, given that he killed two of them, in two separate incidents.
If he has the moral right to use lethal force to defend himself against a threat to his person, with imperfect information, without waiting a year for a court to split hairs over who was and was not at fault, so do they. He is incredibly lucky that Grosskreutz gave the question of killing a man more thought than he did.