I will venture a guess . . . jealousy. Others have things which they don't have and they are internally angry (they might not even sense the anger); their meanness to others is the outward expression of frustration that their own lives are not quite what they would wish for . . . . .
I may be wrong, but most of the people I've met who I observe to have a mean streak are self-centered, insecure, and frustrated in their life situation . . . because of that, they bully others to feel better about themselves.
The world is a mean place. Sometimes you have to be mean just to survive in it; some people never learn to turn it off. Some, and i dare say most of all people are stupid, selfish hateful and mean all of the time. When you find a person that doesnt suck you should make friends and stay in touch. They are rare
Lots of reasons, pride, envy, jealousy, selfishness. Some like the way they feel when they make another feel bad... Power. its possible those who behave mean along the way was not loved or they where neglected, or abused, and thus a part of their heart is cold.