Satire depends heavily on the cultural context in which it was made. Charlie Hebdo is certainly a leftist rag, and certainly satire, and certainly understood as such within France’s cultural context. However, there are some universals about satire that people, time and again, forget.
The first and most important thing to remember is that satire can damage just as much as the original offense, and sometimes more. Charlie Hebdo’s satire was about taking some aspect of the news cycle — some politician or celebrity who held racist and sexist views — and illustrating the logical end result of those views. In a context where a great deal of damage has been done by outright propaganda by outright racists and sexists, where “Evil Banker Jew” and “Monkey-Like Black Person” are well-worn tropes, depicting them as though you’re resurrecting the trope in order to scandalize the person who still holds those views is fraught and potentially more damaging to the person who’s damaged by the original racism.
The second thing to remember about satire is that it is a powerful weapon, to be wielded carefully so as to avoid splash damage. Attacking a class — or being perceived to be attacking a class — that is already under siege by society, is “punching down”. Even if you’re trying to shame the person who’s holding an antisemitic or anti-black or anti-woman view, you could very well legitimize or normalize attacks on that class of person by increasing the number of instances where it’s perceived to be acceptable. Increasing the frequency of a meme does not NECESSARILY legitimize it, but it CAN.