Summary: The psalmist assumes the role of God's chosen king, beating back attempts from earthly powers to overthrow him. He asks the other kings to listen to advice, insisting that serving God is the only right way.
Response: If taken the wrong way, some of this does seem mean spirited, such as God Himself laughing derisively at the other kings. But as I wrote above, David (it's more than likely David) offers repentance even to the other nations. I need to accept that mercy, I need to change my life because of it, and it starts by offering that mercy to others around me who need it. Not "who deserve it" (I'm sure none of the other kings took David's offer, but I'm sure none deserved it), but "who need it."