why are some people so mean?

"If you smeeeeeeeeeeell what the rock is cooking"

The meaner someone is, the unhappier they are inside and the less-able they are TO work with their unhappiness and cope.
So they try to repress their emotions, but all this does is build up tension and anxiety and anger .. .then, because they do not have the courage TO face their emotions nor the strength to work with them ... they find someone to "blame" for their unrest and they blow off their tension by being mean.

Remove yourself as much as possible from mean people.
Try not to take it too personally .. because it is not YOU .. it is their fear of their own pain that makes them this way.
Try to develop some compassion inside your heart .. because if you don't, you too might turn YOUR pain into meanness and become like them.

they have low self esteen

Because they know they're a waste of oxygen

you mean stupid

Because they want to shake that boo-tay!