In Psalm 41, it seems David gets distracted and each verse (stanza) is different. Actually, they are all tied together. If we take an overall view of this Psalm, we can see that David is speaking of being merciful just as God is merciful in different contexts. In Matthew 5:7 Jesus states:
Blessed are the merciful, for they shall receive mercy.
First, David exhorts us to be merciful and come to the aid of those who cannot help themselves. This is repeated in James 1:27. All throughout the scriptures we see God’s encouragement to help those in need: widows, orphans, the spiritually dead, the religious oppressors, the idol worshippers…the list goes on. In other words, we are called to serve the poor (note: this does not address just physical or financial poverty.)
Next, David asks for mercy. When his enemies rise up against him and he is not well, he asks God to be merciful. He is bold enough to ask for it and humble enough to expect God to show it. Even when David’s best friend turns against him (think about the story of Job!) God is there and will not turn his back on him. The same holds true for us!
In the final stanza (vs. 10-12) David talks about “repaying” his enemies once God restores him. Let us not be quick to think that this means when God restores us we can persecute, punish, and assault those who mistreated us. We know that vengeance belongs to the Lord (Romans 12:19-20). Instead we must love them. We must have mercy upon them in their sickness (i.e. sinfulness) just as God is merciful towards us. In doing so, we live out verse 13: blessed be the Lord.