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Except nations do it anyway, regardless of the message it sends to potential exchange partners. Mazen Faqha was released by Israel in a prisoner exchange, and was later assassinated.

It serves as a deterrent if it sends the message to your own people that treason will never be forgiven, regardless of a pardon and exchange. And that is a philosophy to which Putin subscribes – he once said, "Enemies are right in front of you, you are at war with them, then you make an armistice with them, and all is clear. A traitor must be destroyed, crushed."