> There are no natural contracts.
What really matters is why we should inform our thinking with how animals treat each other.
Frankly, this has little to do with the original post - I just made my original comment in response to someone saying "don't kill" is an example of a natural contract.
On the other hand, I do find my aversion to killing rather natural in the sense that I needn't justify too much beyond my own instinct (and, if you should know, ethics as instinct is the subject of research with promising theories, but I'm not even talking about that).
One of the natural rights protected by the Common Law is the right to enter into contracts and have the terms of those contracts enforced in a court of law. But a contract cannot abrogate a natural right, or such contract is null and void. Thus you cannot enter into a contract to take the life of another person.