Christopher Hitchens can put it much better than I could.

Essentially, the free speech of someone or about something you find abhorrent is the most important kind of free speech, and should be protected more fiercely than any other kind of speech.

The question of who to make arbiter of what can and cannot be said is basically unanswerable. Free speech must also be considered our free right to listen to said speech. When we deny someone else the right to speech, we are also denying our right to hear their speech.

It seems like a far better idea to make all speech open and let each person decide what to hear and say for themselves.

The idea of limiting harmful speech is one of those idealistic, in-a-perfect-world things where we can be absolutely certain of what will and will not cause harm. It's not something that's reasonably knowable or achievable by society. Hitchens makes a great point about this when he questions the idea of restricting someone from shouting fire in a crowded theatre (interesting hearing the origin of the phrase). How do we know for certain that there isn't a fire that only this one person can see at the moment? Does it seem more rational to immediately punish this person for shouting fire? Or to wait and see after the fact, and if the person is shouting fire when there doesn't appear to be any fire, and the person had no rational reason for shouting fire, then we simply stop listening when this person shouts fire.

In the story of the boy who cried wolf, the villagers don't attempt to change their entire shepherding system to prevent a false positive after the first time the boy cries wolf. They continue to respond until it becomes obvious that this specific person is not trustworthy, they do not decide that the entire system is not trustworthy, or that new rules should be put in place...

When it comes to being pragmatic about mitigating harm (Charlie Hebdo), I think it's dangerous to insert a causal relationship for:

fewer Muhammed cartoons --> less violence from Islamists

There is not much clear evidence out there that this is the case, except for the somewhat muddled message from the Islamists themselves ("stop doing this or die," basically). The danger here is that you are actually giving them hard evidence that there is a causal relationship for:

threaten some violence --> carry out some violence --> threaten worse violence --> get what you want

As it applies directly to what you said:

I don't think the idea of free speech is outweighed by the cost of human life

Who says those two things are sitting on opposite sides of a scale? Seems like a false dichotomy to me.