In his fascinating Iraq war account Generation Kill, Evan Wright describes his experiences as a reporter embedded in one of the first Marine units to invade Iraq in 2003.  

One lieutenant, describing the "Gen X" and "Gen Y" youngsters fighting in Iraq, observed that during World War II, when the Marines hit the beaches in the Pacific campaign, "a surprisingly high percentage of them didn't fire their weapons, even when faced with direct enemy contact.  

Not these guys. Did you see what they did to that town? They f*****g destroyed it. These guys have no problem with killing." 

Our sin nature notwithstanding, any typical human being has exceptionally strong inhibitions where taking another life is concerned. This internal restraint can be subverted by a process of self-seduction in the service of some illicit design; it can be undermined by severe emotional or psychological trauma.  

For those in the military, it is nullified through patient, deliberate indoctrination – and even then, the psychological impediment to homicide still re-asserts itself for many in the military. 

But "Generation Kill" includes more than a few young men produced by a deeply nihilistic popular culture who have exceptionally few compunctions about killing. 

When they are recruited into law enforcement, they will retain both the mindset and muscle-memory of trained, remorseless killers. 

Rubicon in the Rear-View, Part I: Militarizing the Police