We tend to explain behavior in terms of internal disposition, such as personality traits, abilities, motives, etc. as opposed to external situational factors.
"Beware of the fundamental attribution error of attributing other people's behavior to intrinsic characteristics rather than possible external factors. Such bias has caused many unnecessary misunderstandings in relationships.
Famous Milgram 'electric shocks' experiment drew wrong conclusions about evil individuals says psychologists - Science - News - The Independent. It never helped the ones who went to the electric chair either.