Some aspects of criticism go beyond words and demand justice or physical judgment. These are not to be dealt with individually. Instead, God has established governments, who appoint judicial systems, to deliver justice; and, God often uses other more direct methods such as bringing natural disasters, sickness or even other nations to bring about justice.

The entire Canaanite war was premised on restoring justice to the land for the sin of the inhabitants (Genesis 15:16). David praises God that God is a God who judges the earth—when the wicked become a bath of blood for the godly (Psalm 58). But he is not bloodthirsty—David even calls that same justice on himself if he is found guilty (Psalm 7).

So it is perhaps surprising that while David readily participates in God's judgement by killing Philistines, he is not happy to kill Saul, even though God gave him a blank cheque to do as he pleased (1 Samuel 24:4), and Saul deserved to die (for attempted murder of David, and actual murder of the priests and women and children of Nob, 1 Samuel 22:6-19). David doesn't see this opportunity as one to exact revenge, but as one to draw out repentance and renewed trust. And Saul does repent (1 Samuel 24:22)! As happens, sin overtakes us and we repeat sins, and Saul takes after David yet again (1 Samuel 26). Amazingly, David does a very similar thing and seeks another repentance—which is given (26:21).

For David, leaving vengeance to God meant that God himself would strike Saul down, perhaps through circumstance or perhaps through battle (26:10). Saul is eventually killed in battle (1 Samuel 31), and David mourns this loss (2 Samuel 1:11-12; 17-27)—he had replaced a heart of vengeance with a heart of repentance and trust. He trusted that God could redeem and turn people around, and he trusted that God would exact justice at the proper time.

This principle is affirmed in the New Testament. Paul says of God, "vengeance is mine", (Romans 12:19b, quoting Deuteronomy 32:35). He elaborates, saying also: "repay no one evil for evil" (12:17) and "never avenge yourselves, but leave it to the wrath of God" (12:19).

So that's what we shouldn't do, and so what do we replace it with? David shows we replace vengeance with redemption, Paul says that we instead repay evil with good (Romans 12:20-21), and Jesus says "love your enemy and pray for those who persecute you" (Matthew 5:44), not resisting the evildoer (5:39). This demonstrates God's mercy and grace (5:45), and when we demonstrate something of God we are witnessing about Him, which is our main reason for this first life we live.