Psalm 34 began with a devotional song of the saved. The ‘song’ turns to the sermon in v11`.
Following David’s deliverance through feigned madness among the Philistines, many who were in distress, in debt, or in discontent gathered to him at the Adullam cave (1Samuel 22:1-2). It’s reasonable to think that David taught these men his own recent lessons of faith, including ‘the fear of the Lord’ v11. David is saying that the fear of the Lord is doing right, that is, that it involves obedience. V12-17

David’s instruction of his men was very much in light of the Old Covenant, by which he and the rest of Israel related to God. Under the New Covenant God’s blessing is in Jesus Christ and received by faith, not by our own obedience. However, to teach men how to live and how to die is the aim of all useful religious instruction.

David continued to instruct his men, teaching them about the watchful eye and attentive ear of God upon His people. This was another aspect of reward to those who lived the obedience described in vs13-14. David’s testimony is one of rescue from trouble. This teaching from David was wonderful for the men to hear. They, being in debt, distressed, and discontent – were likely those with a broken heart and a contrite spirit. They were, however, objects of God’s favour and salvation, not His scorn. Like a bird with a broken wing, these men had a broken purpose in life. Broken hearts think God is far away, when in fact He is most near them – if we would but turn our face toward Him. NB v19. C.f also the Beatitudes, Matt 5:3-11
According to the Gospel of John, David in v20 spoke not only of his own experience. He also spoke prophetically of the Messiah to come, Jesus Christ. See John 19:36.

C.f. v22 with Paul’s words many centuries later in Rom 8:1. Even under the Old Covenant, David knew something of this freedom from condemnation.