Hello Annie Lannie: I noticed this is your first post here on PC. Welcome to Psych Central.

You asked why a person is mean to others. I doubt I really have the answer to that. I suppose there can be a variety of possible answers depending on the person & the situation. For some people there may be a genetic component to it. It may also have to do with the person's upbringing. Perhaps they were abused growing up & so, now, they take their anger out on others. One really never knows for certain what may be going on with another person.

Here's a link to an article, from Psych Central's archives, that offers some possible reasons why a person might be angry all the time:

Angry All the Time for No Reason? This Might Be Why

I hope you find PC to be of benefit.