The Apostle Paul was not necessarily a nice guy to his enemies, as judged by modern Christians. He seemed to hold deep grunges and wished destruction on his enemies. Often quoted is his verse to pray for one’s enemies:
Rom 12:14 Bless those who persecute you; bless and do not curse…
Rom 12:17 Repay no one evil for evil. Have regard for good things in the sight of all men.
But Christian often ignore the verse a couple lines down (which is quoting Proverbs 25):
Rom 12:20 Therefore “IF YOUR ENEMY IS HUNGRY, FEED HIM; IF HE IS THIRSTY, GIVE HIM A DRINK; FOR IN SO DOING YOU WILL HEAP COALS OF FIRE ON HIS HEAD.”
Goofy people translate this as “make him a nice fire to warm his face”. But does this make sense: If your enemy is hungry or thirsty, feed him or give him drink because that will build a fire to warm his cold face. The point of Proverbs and Paul’s point is that if you are kind to someone and they still reject you, they will suffer. Proverbs might be more geared to the here and now: being nice to people who hate you often makes them stew with more hate. Romans is probably using Proverbs to deal more symbolically with the afterlife: being nice to someone now leaves them with no excuse to avoid future judgment.
We see Paul using this elsewhere:
2Ti 4:14 Alexander the coppersmith did me much harm. May the Lord repay him according to his works.
This is the most concise calls for vengeance someone can make. Paul held a deep grunge against Alexander (possibly the same as Acts 19) for endangering Paul and undermining his gospel.
People should pray for their enemies and treat them well. But God will be our enemies’ ultimate judge, and there is nothing wrong with wishing for judgment.