The keyword there is "active". KR was running to where the police were, saying he was running to the police [to turn himself in] and the mob chasing him attacked him. Within sight of the police line. They were in no danger from him and were not stopping an active shooter fleeing a scene attempting to escape justice.
Look at what happened with Gaige Grosskreutz. Gaige comes running basically besides Kyle, asking him "You shot someone?" Kyle replies "I'm going to the police". So what does Gaige do next; he drops back a bit behind although still perusing Kyle and starts drawing his illegally carried pistol out of his backside holster. This all before Kyle had stumbled (after being hit twice in the back of the head) and shot at Jump Kick Man (that we now know the identity of, and apparently the prosecution knew the whole time of the trial but with held the identity), and then shot Huber after being hit with his skateboard and Huber attempting pulling the rifle away.
That said however I'm not 100% against the "stopping an active shooter" and give some credence they may have believed that at the time in the heat of the moment. With benefit of hindsight and all the recorded evidence it doesn't fit but in a chaotic environment like it was some benefit of doubt is granted. However that doesn't remove Kyle's self defense right. Said before that self-defense is not zero-sum or mutually exclusive; two parties can both be acting in self defense against each other depending on circumstance.