Recently I saw an interesting e-mail on the VVAW list. Of course the basic reason is lack of control, and insufficient leadership and character. The very existence of war is an atrocity. Whatever reasons and justifications a society has for using physical force are expanded and amplified in wartime.
Violence is only lastly physical, though.
Mental preparation comes first. Ignorance fuels abstraction. Young men in groups, with or without weapons and official authorization, are capable of terrible things anyway. Think skinheads, soccer hooligans, and the NYC groups that harassed young women at a street fair. Many VC died - along with countless Vietnamese civilians who weren't engaged in acts of war but were simply in the wrong place at the wrong time.