It is simple fear.
If the religious (fanatics of any faith or denomination) admit to an over reaction, or leave a "blasphemy" unpunished then they have as much as admitted that they have allowed themselves to be fooled in to a life time of servitude, worship, study and belief that is of no consequence to most of the rest of the world.
They have denied themselves worldly pleasures for nothing, been obliged to part with large sums of money they could have put to better use, allowed themselves to be bullied in to relationships they did not want, and been persuaded to hate those they did not wish to hate.
Therefore defense of the indefensible is there only option, discussion is worthless and foolish for they have no evidence or logical argument so they fall back on "if not by discussion, then by the sword" in order to feel justified at their total waste of a life.

Charlie Hebdo paid a tremendously high price for doing something had a legal and moral right to do. I applaud their position in republishing the cartoons. And, yes, all religions should be open to satire.