If you're wrong about that, we'll win the War on Islamo-fascism by continuing to silence martyrs.
Very simplistic answer.
What you call "martyrs" are merely foot soldiers.
If you are going to kill somebody, it is far more effective to kill the radical cleric than poisons a thousand young minds or prove him a fool to his followers than to wait ten years to kill the product of such a poison.
The poison has already been unleashed by this man's arguments and future generations will see that your response was neither to kill him nor prove him wrong but to merely prohibit even bringing up the subject.