I think this response from the New Yorker framed it remarkably well:
But the murders in Paris were so specific and so brazen as to make their meaning quite clear. The cartoonists died for an idea. The killers are soldiers in a war against freedom of thought and speech, against tolerance, pluralism, and the right to offend—against everything decent in a democratic society. So we must all try to be Charlie, not just today but every day.