No one has a right to kill an enemy except when he cannot make him a slave, and the right to enslave him cannot therefore be derived from the right to kill him.
 Jean-Jacques Rousseau, The Social Contract (1762). copy citation

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Author Jean-Jacques Rousseau
Source The Social Contract
Topic killing right
Date 1762
Language English
Reference Of the Social Contract, or Principles of Political Law, Book I
Note Translated by George Douglas Howard Cole
Weblink https://en.wikisource.org/wiki/The_Social_Contract/Book_I

Context

“The right of conquest has no foundation other than the right of the strongest. If war does not give the conqueror the right to massacre the conquered peoples, the right to enslave them cannot be based upon a right which does not exist. No one has a right to kill an enemy except when he cannot make him a slave, and the right to enslave him cannot therefore be derived from the right to kill him. It is accordingly an unfair exchange to make him buy at the price of his liberty his life, over which the victor holds no right. Is it not clear that there is a vicious circle in founding the right of life and death on the right of slavery, and the right of slavery on the right of life and death?” source