Having said that, I think, given the right circumstances, everyone is a potential killer. Everyone is physically capable of ending an other man's life. What keeps us from killing everyone whom we disagree with or dont like is our ingrained sense of ethics and moral fibre. This is basically what seperates man from beast. However, put a man in a situation where normal society rules dont apply and he is reduced to his basic animalistic instinct. All animals have their own defence mechanism and each one will react when threatened. This is known as d 'Fight or flight' syndrome. A cornered animal is at his most dangerous as he has nowhere to run. The same with humans. Remove all options of 'Flight' and he will attack. He may even kill.
E.g. Would you not risk killing someone should he be threatening d life of a loved one ? Would you not attempt to stop anyone from killing you , even if that means ending his life in d process ? When i used to train Martial Arts i always went by two Mottos :
We teach you how to fight so you dont have to fight
It's better to be judged by 12 then carried by 6
The two mottos both represent two different approaches (there's Duality !) The first is teaching you self control and discipline and to always walk away from a fight (FLIGHT) and the second is telling you that should the first one not be an option, well, strike to kill, self preservation, better taking your chances with a jury then be killed (FIGHT)
Anyway... its something to think about...and also a good reaason why we should never judge anyone until we've walked a mile in that man's shoes.
Peace