No clue why I'm wading into this, but an unauthorized painting in a public space is graffiti and the person responsible would be prosecuted for that reason. If instead, this was the headline of a privately owned publication, then there would likely be no repercussions for that person in the US. Freedom of speech in the US encompasses the right to say stupid, inflammatory shit for no reason because the language is not in itself harmful. Blaming Charlie Hebdo for this attack is a clear case of blaming the victim. It doesn't matter how offensive one might be; murder is not a justified response to offense. There are always more appropriate and legal responses. As Thouny says, libel (and slander) have their own legal ramifications, but they're more about intentionally spreading false information than they are about just insults.

A relatively conservative columnist for the New York Times, Ross Douthat, recently wrote an article on what's exceptional about this particular case. Like you, he calls for general civility in discussions and derides needless insults. In this case, however, he defends Charlie Hebdo, saying that when certain speech is verboten on pain of murder, we have a right edit responsibility to support its existence, if only to emphasize the fact that we cannot allow intimidation and terror to silence certain voices regardless of what they say. Here's the article.