Moral relativism
So, if a criminal decides he doesn't agree that his actions are criminal, then we have no business prosecuting him?
What exactly does "accepting the social contract" mean? Who writes the "contract," and based on what?
Principles don't have to have a religious basis. Ayn Rand founded an entire consistent philosophy of morals and principles without any religious basis at all. But you do need objective morality somewhere in the mix.
There are plenty of absolutes in the world. Alive vs. dead is just one, and a lot of morality can be derived just from that one.