I was reading an article in the current Scientific American (I think) about a study of the correlation between self-esteem and aggression. The thing I took away from the article was that people with unusually low self-esteem may lash out spontaneously and violently, while people with unusually high self-esteem are also likely to lash our violently, but they're more likely to do so deliberately and purposely, or as the result of a challenge to their superiority.

It was called "Self-Esteem, Narcissism and Agression".

A large part of it may just be having the right balanced between awareness of your own falibility and awareness of your own value.