That is true, but people are capable of holding both views at the same time. Soldiers on the battlefield go out and do horrific things to enemy soldiers and civilians, and come home and are loving fathers and husbands who wouldn't hurt anyone. Or how many times have people been caught for horrible crimes and all their friends and neighbours say it isn't possible because they're the kindest and most helpful people they know.

This isn't a matter of "some people are capable and some are not". It's a case of "most people seem to be able to set aside someone else's humanity to do horrible things"