Rom 12:20-21, Mt 5:44, Lk 6:27-28, Lk 23:34
In the Old Testament, the prophet Elisha did not kill the Syrian army, but fed them and let them go. (2 Kings 6:20-23)
2 Kings 6:20 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. 21 And the king of Israel said unto Elisha, when he saw them, My father, shall I smite them? shall I smite them? 22 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. 23 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.
Jesus told us to love our enemies and pray for them. (Matthew 5:44, Luke 6:27-28)
Matthew 5:44 But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;
Luke 6:27 But I say unto you which hear, Love your enemies, do good to them which hate you, 28 Bless them that curse you, and pray for them which despitefully use you.
Jesus forgave his enemies who killed him. (Luke 23:3-4)
Luke 23:3 And Pilate asked him, saying, Art thou the King of the Jews? And he answered him and said, Thou sayest it. 4 Then said Pilate to the chief priests and to the people, I find no fault in this man.