Every single one of the men above was trained by the US armed forces - both physically and psychologically - to kill. Every single one fought in Vietnam. Every single one killed multiple times on their return.
While it’s true that some of the Vietnam vets may have had a ‘killer instinct’ and been tempted by the notion of a state-sanctioned licence to kill, many told police when apprehended that their crimes were the result of their experiences during combat. It’s arguable, of course, that this is merely a useful excuse. The points stands, however, that they each took on board some pretty heavy education in the ‘art’ of murder. Learning not only how to kill, but how not to care.