As I had a just cause, and made it my care and business to deal righteously with God, and with Saul, and all others; so God (who hath engaged himself by his promise to suceour and reward them that are such) was graciously pleased to own me, and to plead my cause against my unrighteous enemies. And because I would not deliver myself from straits and miseries by unrighteous means, namely, by killing Saul, as I was advised to do, God was pleased to deliver me in a more honourable and effectual manner. The cleanness of my hands, i.e. the innocency of my actions and carriage towards Saul, from whose blood I kept my hands pure.
Related Commentaries of Psalms 18:20
Psalms 18:20
20 The LORD rewarded me according to my righteousness; according to the cleanness of my hands hath he recompensed me.