Because he was the source of danger. The test is really simple here (in many cases it isn't, but this is not one of them). To know whether person A is justified

Because he was the source of danger. The test is really simple here (in many cases it isn't, but this is not one of them). To know whether person A is justified in defending himself, ask yourself - if A would do nothing (except maybe run away), would they be in danger? If Grosskreutz did nothing or run away, he'd be in no danger. Rittenhouse was running away, and if he didn't shoot he'd likely be either in a hospital or dead. Maybe shot by the same Grosskreutz - pointing a gun at a person is rarely for just showing him what a cool new gun you have.