Military necessity does not admit of cruelty - that is, the infliction of suffering for the sake of suffering or for revenge, . . . nor of torture to extort confessions.
The death penalty fulfills a preventive function, but it is also very clearly a form of revenge. It is an especially severe form of punishment because it is so final. The human life is ended and the executed person is deprived of the opportunity to change, to restore the harm done or compensate for it.
But your mind is warped by an innate principle of general integrity, and, therefore, not accessible to the cool reasonings of family partiality, or a desire of revenge.
Hath not a Jew eyes? Hath not a Jew hands, organs, dimensions, senses, affections, passions? ...If you poison us, do we not die? And if you wrong us, shall we not revenge? If we are like you in the rest, we will resemble you in that. If a Jew wrong a Christian, what is his humility? Revenge. If a Christian wrong a Jew, what should his sufferance be by Christian example?
Therein lies the defect of revenge: it's all in the anticipation; the thing itself is a pain, not a pleasure; at least the pain is the biggest end of it.
Mind is a duality; it is always split. There is no single point on which the mind agrees in totality. Half of the mind will agree and half of the mind will disagree, and whatever you choose, you are choosing only the half. The remaining half is going to take revenge. The unchosen part, the left over, will wait for its chance to show you that whatever you have chosen is wrong. But it does not matter which part you choose. Choice itself is wrong.