You r not true, when u said there should be a common law. The reoson being there r many believes which according to muslim can be practiceble, but may not be as per general law. But regarding your point about punishing "fanatist" - (A group which cause distruction under the wrong names of religion) - and islam has stated what kind of punishment should be given to them & u believe it or not the punishment by islam to such group, even if he calls himself muslim is more severe that any present law.
