Combat veteran

Through various research it has been established that humans, at least the majority of us, are not capable to kill another human being as it's considered unnatural by the brain. Only a small percentage of the human population find killing another human normal. They are called psychopaths. Exposing your average Joe to such situations (e.g. war) will eventually lead to a number of mental health issues, such as PTSD.

Take over Russia he said. It'll be quick he said.

It is possible to lower that "don't kill" factor by extensive training and making the use of weapons a common thing, which will lead to combat activities being just a muscle memory thing. But all the training won't do much after combat, when a person is alone and thinks of the things he's done.