I wish it was that simple, but alas life never is.
My Dad was a WWII veteran and had nightmares for the rest of his life. He always said war is despicable and that fighting in the war was the lesser of two evils. The greater evil was what Hitler was doing.
A lot of soldiers killing people today are doing so because someone convinced them the military would provide an education, or an income, or something they were lacking. And then they were called to serve and the horror begins.