Dealing with an Evil Professor

I have an abusively mean professor. He is condescending and has awful mood swings. His grading makes zero sense, and most of his assignments seem to have no fluidity. A couple of students, including myself, are considering talking to the head of the department. Do you think this is a good idea? What can I say...gently? He is not tenured...however, his wife is. 

Yes, I would talk to the head of the department. This professor is not acting professionally nor appropriately for the position of power that he is in. The key words you have written: mean, mood swings, condescending, erratic in his assignments and grading, may all be indicative of an emotional or mental issue that he needs to deal with. 

I would write down specific examples of his behavior and even quote several inappropriate remarks he makes in class, the effects on students and put it all into bullet points. Do this over a period of time of a week, two weeks or even a month to show that his behavior is not just a fluke and due to having "a bad day." The more time you spend and the more clear cut examples you can put down, the better your evidence is to present. This will draw a definitive picture of him that the department head can take to his superiors, if need be, or to formulate a plan for dealing with the situation.  

Remember, there is strength in numbers. If one or two students are behind your fact gathering mission, it may be dismissed as sour grapes; but if half a dozen of you or more are all on the same page about his behavior, it is harder to ignore. To bolster your strength about your concern about this professor, you may also want to consider stating the consequences you all are prepared to invoke if this is not looked into and handled properly. If you feel strongly, you may feel you have a duty to warn others who may be thinking of signing up for his class. It is imperative that you stick strictly to the facts and not embellish anything in any way. You don't want a libel or slander situation to arise. 

If this professor has a personality disorder, mood disorder or character pathology, it needs to be assessed, diagnosed and treated. A person in authority should not be allowed to harm others.