"It wasn't his job to do that, and he didn't need a gun to put out fires or give first aid"
First, I didn't ask if it was his job. Second, given how Rosenbaum behaved, it is quite clear that if Kyle was not armed and tried to prevent their destruction, he would have suffered grave injury. So yes, he did need a gun, if, like a normal human being, you take "not suffering grave personal injury" to be a need.