In light of the terrorist attacks in Paris against a newspaper and cartoonists who wrote and published cartoons that mocked Muhammad, I will simply repeat something I wrote in one of my first posts regarding the controversy over a Danish newspaper that commissioned and published some cartoons depicting Muhammad. Part of the issue there was that many Muslims oppose representations of Muhammad. That's less of an issue with the Paris newspaper, since they regularly published cartoons that were intended to be offensive far beyond the mere representation of Muhammad. This only affects points 3 and 4, however, and does not actually affect their main points.
1. It's incredibly ungracious to treat something profanely when many people consider it sacred. It's morally reprehensible to do something for the sole purpose of offending others, especially when it comes to something as close to people's personal sense of identity as their religious beliefs.
2. Nevertheless, they had the right to do it. Free speech, freedom of the press, etc. entails the right to offend. If you only have free speech until someone is offended by what you say, you don't really have free speech.
3. To respond to some cartoons by committing such horrific acts of violence is absurdly disproportionate. It doesn't matter how offensive the cartoons are: the terrorists are not animals only responding to external stimuli. They are human beings, and so answerable to God for their chosen actions, and for choosing to align themselves with evil.
4. The prohibition of making images of Muhammad is not a universally-held doctrine in Islam. Many museums throughout the world, including the Muslim world, have paintings of Muhammad, which have been made by both Muslims and non-Muslims throughout Islamic history. Drawings and paintings and even cartoons of Muhammad -- including, most relevantly, offensive cartoons of Muhammad -- have been made many times before without similar responses. As such, these terrorist acts show all the signs of being a contrived outrage. The terrorists, in other words, used these cartoons as a pretext to express the evil that was already in their hearts. This doesn't necessarily absolve Islam (see here and here), it just puts the responsibility for these wicked acts on those who committed them.