Facing away from you, there's no threat to you. Despite the fact that he has a firearm, if you shoot him in the back, you're fornicated. The 'Code of the West' is still alive, well and just as stupid.
In some states, TX is one I know of for certain, you can use deadly force on behalf of another under the same sorts of circumstances that they could use deadly force on their own behalf.
There is nothing in the deadly force statutes in TX that would indicate that shooting someone in the back would invalidate an otherwise valid claim of legal self-defense.