“People do as they are told, up to and including killing others, if ordered to do so by a superior.”
Hence the US Army, Navy, Air Force, Marines.
The military actively seeks out certain people to do special jobs such as snipers and socom. There’s a not small portion of the population that given the change enjoy inflicting pain and killing other humans it’s just a fact some people are programmed that way. Given the opportunity to hunt the ultimate prey some people jump at that opportunity. For those in the know they would recognize my tag line.