I watched a great movie, The Help, released in 2011 and starring Emma Stone, Viola Davis, Jessica Chastain, and Octavia Spencer.
In the movie, there is a scene in which there is a conversation about how people are mean to maids:
I worked for Miss Jolene's mother till the day she died. Then her daughter, Miss Nancy, asked me to come and work for her. Miss Nancy is a real sweet lady. But Miss Jolene's ma done put it in her will I got to work for Miss Jolene. Miss Jolene's a mean woman. Mean for sport. Lord, I tried to find another job. But in everybody's mind the French family and Miss Jolene owned me.
In this paragraph, I don't get it why 'mean for sport' is another word for "she is really really mean". (I found out the alternative meaning from the Korean official subtitle)
Are there any special idioms for mean for sport? I'd be grateful if someone could provide me the reason. (FYI, I am not a native speaker, and I study English these days)