The Actor-Observer Bias

When people do something unforgivable, we often find it easy to conclude that the wrongdoing is a manifestation of their nature. This allows us to go after them with a vengeance -- when you are dealing with fundamentally bad characters, anything that can undermine them is fair game.
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When we do something wrong ourselves -- drive 60 mph in a 40-mph zone, for example -- we explain our actions in terms of situational factors. We say we are speeding because we are running late, or that we got held up at work. But when we see someone else do something wrong, we are far more likely to link the behavior to the nature of that individual.