Why Didn't She Just Dump Coffee in Her Lap?

A Texas woman was sentenced to 10 years in prison for running over a McDonald's manager when an employee told her she couldn't get mayonnaise on her cheeseburger.
I still haven't figured why some folks get so violent when they don't get their way. Behaving like a spoiled child is one thing but taking it to this level is quite another.
I don't believe this women was suffering a breakdown similar to the Michael Douglas movie "Falling Down" but committed an amoral act of utter selfishness. I would think that a lifetime ban from operating a motor vehicle is appropriate also.