What does Psalm 35:12 mean?

David had been good to those who now sought to kill him. Twice, David could have killed Saul (1 Samuel 24; 26). On one occasion Saul even admitted that David was a better man than he (1 Samuel 24:17). Yet he continued to oppose David, as did his men. Instead of responding to his kindness with good, David's enemies did him wrong. Such ungrateful treatment saddened David deeply.

Essentially, all humanity has repaid God's goodness with evil. God created a beautiful world for mankind to enjoy. He faithfully causes the seasons to come and go in turn. His faithfulness is renewed every morning. He provides harvests and shelter for all. He demonstrated love toward everyone by giving His Son while everyone was lost in sin (Romans 5:8). Nevertheless, mankind has spurned the Lord's love and goodness. Isaiah 53:6 portrays all mankind as turning away from God and going astray. When in His goodness and grace Jesus came to earth to seek and to save sinners, the world rejected Him and ultimately nailed Him to a cross (Matthew 16:21).