Psalm 21 gives us two cases: that of the Lord's friend (the king), and that of His enemies.
For the king who trusts in the Lord, we discover that God answers his prayers and grants his desires. God blesses the king with long life and good possessions, along with abundant happiness and security.
Not so with those who make themselves enemies of the Lord! Their lot is one of suffering and destruction. They will be swallowed up in the fiery wrath of God - they and their offspring! Their destiny is one of failure and flight.
It is clear Whose side we ought to be on!