In order to adequately define mental "illness", it is incumbent upon the label itself and upon those who proffer it to infer a baseline reference of normality, if such a thing is possible.  

Apart from the pejorative use of the terms mentally ill, crazy, insane and so forth to denote behavior that an observer finds distasteful, the notion of mental "illness" has been abused by many e.g. Russian gulags among other ideological prisons.  In particular, the medical establishment via every iteration of its DSM purports mental "illness" categories for nearly every human being alive to the point of ridiculousness.  Not long ago, homosexuality was a mental "illness" per DSM.

Is there a clear boundary between mental "illness" and a mere difference of opinion?  Is a reaction to social or personal injustice, perceived correctly or not, a mental "illness"?  The term should not be used lightly or flippantly by anyone, professional or otherwise and it certainly must not be used to excuse violence.