Which he didn't use against the first teen. He threatened the use of deadly force, yes. He was also facing a person with what he presumed to be a gun in his hand which would be valid to assume presented a deadly threat until the youth disarmed himself.It might be reasonable to intervene, but it was not a proportional response. You cannot use deadly force to defend against non deadly force.
I think it's quite a gray areas to try and argue that it isn't reasonable to point your gun at someone with a gun in their hand... when all they have to do is raise it before you can raise yours. In that case the prosecution would have to argue that he used a disparity of force. Two people, both with guns in their hands, regardless of where they are pointing, is a pretty hard argument to make a disparity of force claim.