Simple Answers

Why do Muslims fly into bouts of fanatical, violent rage whenever someone subjects their revered Prophet to criticism or gentle (even not so gentle) mocking? Well...

Know that all who curse Muhammad, may Allah bless him and grant him peace, or blame him or attribute imperfection to him in his person, his lineage, his deen [religion] or any of his qualities, or alludes to that or its like by any means whatsoever, whether in the form of a curse or contempt or belittling him or detracting from him or finding fault with him or maligning him, the judgement regarding such a person is the same as the judgement against anyone who curses him. He is killed [emphasis added] as we shall make clear. This judgement extends to anything which amounts to a curse or disparagement. We have no hesitation concerning this matter, be it a clear statement or allusion.

The same applies to anyone who curses him, invokes against him, desires to harm him, ascribes to him what does not befit his position or jokes about his mighty affair with foolish talk, satire, disliked words or jies, or reviles him because of any affliction or trial which happened to him or disparages him, because of any of the permissible and well-known human events which happened to him. All of this is the consensus of the 'ulama' [scholars] and the imams of fatwa from the time of the Companions until today." ~ Qadi Iyad (d.1149)