To Christians, this Psalm is prophetically messianic. Jesus himself identified David as writer and interprets its meaning in Matthew 22:41-45; Mark 12:35-37; Luke 20:41-45. David, through the Spirit speaks of "One greater than he".  Jesus challenges the arrogance of the religious leaders and embraces the acceptance of the ordinary folk. 

God, The Lord, directs God's Son, my Lord..."to______________________________", the seat of power. (v.1)
God directs that the Son "to _____________________________". (v.2)
God appoints the Son as priest forever. (v.4)
God describes the future battle the Son will wage and win.

God is the true Sovereign 'King-maker". Jesus the true Savior King.  

The dispensing of God's justice will result in execution of heads of nations.

The psalmist prays to the God of his praise. David often experiences the effects of lies: betrayal of family, friends and slander of adversaries, enemies. It is unclear whom he is referencing here.  David can be considered analogous to concepts of Truth, Right, Life, Good,  even constitutional Democracy and more.  

Vv. 1-5  (nkjv) What are David's "they(s)"? His reasons.

Vv. 6-20 (nkjv) What are David's "let(s)"? His requests.

Vv. 21-25  (nkjv) How does David see himself? His reflections. 

Vv. 26-31  (nkjv) How does David see his God?  His revelation.

American democracy is precious yet fragile. God save our democracy!