Psalm 109

David stands accused......falsely accused.  His accuser apparently was once his friend, for David has shown him love, which has been rejected.  He has indeed prayed for this enemy, with fasting, and has still been rejected.

First, David, as usual, recognizes God's sovereignty and involvement.  He is not distant.  He is addressed as the "God of my praise."  David has known God's hand in his life, and he knows where to turn for help.  Rather than take vengeance himself (perhaps he can't due to his physical state and or the multitude of his enemies?)  he relies upon God to be His defender and to prosecute the wicked.

The first few verses sound like a pitiful cry to me.....deep grief as he pours out his complaint, culminating in verse 5:   "Thus they have rewarded me evil for good, and hatred for my love."

This seems to be the turning point for David's emotions.  It almost seems like his attitude turns into a very human, "it's not fair!", and he launches into a very harsh curse on his enemies.  In short, he asks God to set wicked, unfair judges over them just as they have judged David unfairly.  Let them be treated as they have treated others, receiving cursing and lacking blessing.  Let them be needy and poor, hounded by others like they hound others.  Ultimately, let their family be utterly cut off....with no posterity to carry the family name, so that they will be utterly forgotten in this earth.

Then, after this show of intense negative emotion.....he returns again to a more gentle prayer.....appealing to God's steadfast love.  "Show Yourself, " he says....may all see that You have done this....may all see and know Your justice.

In the end, God's sovereignty is key.  No accuser can stand against Him.  Indeed, God will supplant any accuser against His own.
~~~~~~~~~~~~~~~~~~
It seems inconsistent for David to pray such a harsh prayer, and for this to be accepted.  After all, isn't God love?  Are we not to love our enemies?  And do good to them?

David did do these things.  But remember, he is human, not divine, and is prone to all the depth of human emotion.  

Was he wrong to utter such a prayer of cursing?    I haven't read up on commentaries regarding this, but this is my oppinion:   I think not. David had gone the extra mile, loving his enemies and doing good.  They refused him.  All of the elements of cursing that he expressed are exactly the things God Himself has said about His enemies as some points.   He does not hide that fact that sin will be judged.

Bottom line?  unconfessed sin will not be tolerated by God.  Judgement will come, or for those who put their faith in Christ, it has already been spent on Christ at the cross.