Nice dodge. But if you can't figure out who is not allowed to defend themselves according to some psychological test, then you can't defend them if their allowance to defend themselves is taken unjustly.It's not the wife beater fallacy. The wife beater question explorers the underlying assumptions we make without even realizing we make them. If you ask me if I have stopped beating my wife, the truthful answer is no. What you do with that answer reveals more about you than about me.
As to my question, it is a real world question. There has been at least on case in the news where a doctor raised concerns about patient with police that went ignored. The patient later did use a gun to kill people. There are questions regarding the mental condition of the Aurora theater shooter who was under a psychiatrists care. I saw an article recently where, I believe it was Klebold, was sociopathic. In the real world these things do happen. I can't give you the name of the next killer because I am an engineer and I am not trained to recognize, let alone treat, mental disorders. I would hazard to guess that there is more than one doctor that has a patient that they are more than just a little concerned about.
So, if you knew that a person had a mental predisposition that would make them highly likely to start killing people, would you support their right to purchase all the guns they want?
And if you can't identify them, then everyone is subject to a removal of their rights with your blessing even if there is no reason to suspect they will commit a mass murder.