Lt. Col. Dave Grossman
Credit: John Midgley

One morning in October 2016, Lt. Col. Dave Grossman was in a Red Lion Hotel conference room in Sacramento, California, preparing to speak to a group of state troopers about what it’s like to kill.

Grossman, 60, is a former West Point psychology professor who’s spent much of his career studying killology — his term for the psychology of taking a life. Among the military and law enforcement, he’s a revered figure. His first book, On Killing, is part of the curriculum at the FBI academy and on the Marine Corps Commandant’s Professional Reading List. Its follow-up, On Combat, is probably best known for his assertion that people can be divided into three groups — sheep, wolves, and sheepdogs — and it’s the sheepdogs, “blessed with the gift of aggression,” who are responsible for protecting the sheep from the wolves. The analogy has been adopted by various military and gun-rights groups; in Clint Eastwood’s American Sniper, the father of Navy SEAL Chris Kyle gives […]

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