Besides the nature of man, also the nature of his environment should be considered next.
Suppose there are so few exploitable resources on the island with Crusoe and Friday that only one man can survive on it for any length of time. Then it seems that the two men do have a reason to fight to the death. It may be unjust, but it is at least utilitarian, as the victor will get to live, as opposed to both dying from hunger.
Alternatively, in a land of Cockaigne, Crusoe will not miss Friday even if he dies.
These problems are avoided in the real world where there prevails moderate scarcity.
That a world of super-abundance or extreme dearth will not serve up the right incentives for social cooperation does not give the lie to this fact.
Under moderate scarcity, the forces of life, sustaining and nurturing man, and the forces of death, adversarial to him, challenging him to fight and improve his conditions, are in a rough balance that is most conducive to economic development. Insofar as the world contains plenty of moderately scarce environments, it is wisely designed.