Because he is in a highly volatile area with extremely aggressive people? He went there to help his friends, from what I recall. They had stores there. Honestly, this sounds to me like a lot of victim blaming. It's like asking "why was she dressed like that during the night" to a rape victim. FFS, what does it matter at the end of the day when it is clear as day to see he was defending himself? He didn't shoot first or something, he reacted to what other people did to him.
You argue that a 17 years old can patrol the streets with a rifle and that is totally nuts, but at the same time - the whole situation was nuts. What were these rioters even doing burning buildings, stores, stealing and vandalising their way through the streets? It's not like Kyle went out on a stroll to the mall with his gun.