101:3-4 More specifically David promised God that he would guard his life and his court from sin. Worthless or vile things are things that make no positive contribution to godliness. As God, he professed to hate departure from the right way. A perverse heart means a crooked or twisted affection, namely one that turns away from the straight path of rectitude.
101:5-6 In verse 5 David promised to deal severely with even minor deviations from holiness in others' lives. This expressed his strong allegiance to righteousness.
Positively the king promised to reward people who were faithful to God. He wanted to surround himself with godly people in his court.
101:7-8 Professional competence was not sufficient to qualify a member of David's staff for service. His courtiers also needed to maintain fellowship with God and walk in His ways. The king would not tolerate lying. Moreover he would extend his requirements to all the people who lived in his kingdom. In his daily administration of justice he would cut off the wicked who practiced iniquity. Cutting off might be in death, but it could also mean ending their present course of life by sentencing them to some penalty.
Why did David tolerate a wicked man such as Joab in view of this prayer? Obviously David went back on this promise to God both in his personal life and in his choice of government leaders to some extent. Nevertheless this commitment to holiness is an admirable model for all of God's people. Perhaps David wrote this psalm early in his reign.
