Your counter is very weak. Yes...If I truly embrace the teachings of Aikido, I would try to subdue a madman with a gun without inflicting overkill. I've seen Aikido techniques that disable an armed gunman that turn the tables on him without extreme measures.
I am paraphrasing Alan Watts, but this whole universe depends on each and every individual. It depends on every gnat, and every vibration of every gnat's wings. If one can cherish even the smallest of creatures, then isn't it also important to respect human life to one's own capability?
I think this fundamentally a flawed and unrealistic interpretation of O-Sensei's message and intention.
Yes, Aikido techniques can be used to prevail without serious injury over an attacker if ones skills exceed, by a fair amount I think, the skills of that attacker.
But it is a myth without foundation that this is what happens i Aikido, as a martial art. A martial art, as opposed to some system of self defense, is about an encounter with an opponent who is trained. At least that was always the assumption when the term "martial arts" was coined.
O-Sensei made a couple of statements on this subject. One was that, if you wrote the character for life and the character for death on two sides of a sheet of paper, that was how much separated the outcome in a real martial encounter. In other words, one or the other combatants is dead.
The other statement is along the same lines... He said that the reason that there is no competition in Aikido is because there is no way to do it safely. If there is real contention, there will be injury. He was opposed to sportification, meaning the introduction of rules to allow competition, because he felt that Aikido was a practice that was about being in accord with the natural forces and environment around one, in which there is no real separation between you and anything else, including the attacker. Artificial "rules" have nothing to do with that.