Laws and the justice system are just tools we use to enhance the survivability of the society and the people in it.You might want to consider that it's how we define murder that is the differentiator. You don't consider laws objective, do you?
What is IMMORAL?
Contrary to good morals; Inconsistent with the rules and principles of morality which regard men as living in a community, and which are necessary for the public welfare, order, and decency.
Law Dictionary: What is IMMORAL? definition of IMMORAL (Black's Law Dictionary)