Romans 12:19-21 NET

19 Do not avenge yourselves, dear friends, but give place to God’s wrath, for it is written, “Vengeance is mine, I will repay,” says the Lord. 20 Rather, if your enemy is hungry, feed him; if he is thirsty, give him a drink; for in doing this you will be heaping burning coals on his head. 21 Do not be overcome by evil, but overcome evil with good.

If we repay, or take justice into our own hands, we risk becoming the oppressor. We risk beating the evil of our enemy by being more severe or untrue or unjust than they were. When that happens, we take away the room for God's justice, because He always takes the side of the persecuted or oppressed underdog.

1 Corinthians 6:7

To have lawsuits at all with one another is already a defeat for you. Why not rather suffer wrong? Why not rather be defrauded?

He is saying that it would be better to be defeated than to risk man-made justice that threw dirt on God's face.

How many times do you get in a fight (maybe this just happens between husbands and wives?) and afterwards you see that it was an argument over less than $10? After the fact, don't you ever think "Man, I would totally pay $20 for peace and reconciliation right now!"

Romans 12:19-21 NET

19 Do not avenge yourselves, dear friends, but give place to God’s wrath, for it is written, “Vengeance is mine, I will repay,” says the Lord. 20 Rather, if your enemy is hungry, feed him; if he is thirsty, give him a drink; for in doing this you will be heaping burning coals on his head. 21 Do not be overcome by evil, but overcome evil with good.