If there is an ethical use for a weapon of mass destruction, some individual ultimately is responsible for letting it loose. Thomas Ferebee had to decide whether it was him pulling that trigger or not. That someone else would do it if he didn't was a factor, sure. But it will come down to that at some point: some individual has to say "yes I will use this weapon."

I can't say i find Mr Ferebee's action unethical, given what I know of the situation.

https://en.wikipedia.org/wiki/Thomas_Ferebee