I have heard that in Islamic law a murderer can be pardoned by the relatives of the deceased and left without punishment. Under this system wouldn’t it then be possible for the two parties on either side of the family to conspire to kill: one partner killing the relative, and the other pardoning it?

In all cases of murder whether the aggrieved party forgives the aggressor or whether it insists on equal retaliation, the matter must be brought in front of the State. No settlement can be made between the two parties without the State being legally informed of it. This is the basic principle in this regard. Keeping in view this principle, a State will always be informed of what is going on in a particular case. If in some cases it smells foul play from both sides, it has the authority to annul any settlement and do what is required for justice.