As soon as they entered Samaria, Elisha said, "O Lord, open the eyes of these men so that they may see." The Lord opened their eyes, and they saw that they were inside Samaria. When the king of Israel saw them he said to Elisha, "Father, shall I kill them? Shall I kill them?" He answered, "No! Did you capture with your sword and your bow those whom you want to kill? Set food and water before them so that they may eat and drink; and let them go to their master." So he prepared for them a great feast; after they ate and drank, he sent them on their way, and they went to their master. And the Arameans no longer came raiding into the land of Israel. (NRSV 6:20-23)
And it came to pass, when they were come into Samaria, that Elisha said, Lord, open the eyes of these men, that they may see. And the Lord opened their eyes, and they saw; and, behold, they were in the midst of Samaria. And the king of Israel said unto Elisha, when he saw them, My father, shall I smite them? shall I smite them? And he answered, Thou shalt not smite them: wouldest thou smite those whom thou hast taken captive with thy sword and with thy bow? set bread and water before them, that they may eat and drink, and go to their master. And he prepared great provision for them: and when they had eaten and drunk, he sent them away, and they went to their master. So the bands of Syria came no more into the land of Israel. (KJV 6:20-23)
Elisha doesn't view these Arameans' lives as expendable. It would be unjust to kill them after capturing them. They need to be treated fairly. Hey, unless you're fighting each other on the battlefield, you'd best be polite and enjoy a feast together.