I think there is a difference between murder (especially mass murder) aND being in a war self-defense situation.
I have no doubt that if provoked I would defend my family, and I could kill someone. I also wouldn’t call it murder. Murder is different than killing.
However, I have no doubt that I would never plan an attack on innocent people.
Your last sentence provides the key to understanding my position.
“... an attack on innocent people.”
All it takes is the individual to justify to themselves that the target group of people are no longer innocent. Guilt by association is a common tool / justification used to excuse mass murder. All Jews, all japs ... etc. The NAZIs did it to the jews, and to a lesser extent, the US did to the Japanese-Americans in the use of the internment camps. Many times native Americans attacked and massacred whites and vice-versa.
My pastor once told me that “Hurt people, hurt people”. Given enough hurt, people will often strike out and then justify their actions afterwords.