What does Psalm 18:21 mean?

Obedience to God's commands is a sign of someone who loves and honors Him (John 14:15). David testifies in this verse that he has kept the ways of the Lord and did not wickedly depart from his God. David can't be credited with perfection (1 Kings 15:5), but his habit was to follow God with sincerity (1 Samuel 13:14).

This commitment to godly principles was very different from that of the previous king, Saul. When Saul was selected to be Israel's king, he was humble. He said to the prophet Samuel: "Am I not a Benjaminite, from the least of the tribes of Israel? And is not my clan the humblest of all the clans of the tribe of Benjamin?" (1 Samuel 9:21). When the day came to crown Saul, he was hiding (1 Samuel 10:22). However, late in Saul's reign he lost his humility and disobeyed the Lord's command: to utterly slay the Amalekites and their livestock. As a result of his disobedience, Saul was rejected by the Lord to continue as Israel's king (1 Samuel 15:26–29).

Jesus said, "Everyone who exalts himself will be humbled, and he who humbles himself will be exalted" (Luke 14:11). Saul and David show clearly what happens when humility is either absent from or present in a person's life.