This sounds completely backwards, but it is just how God works.
Proverbs 25: 21 & 22 says, "If your enemy is hungry, give him food to eat; if he is thirsty, give him water to drink. In doing this, you will heap burning coals on his head, and the LORD will reward you."
Let me explain what I think this means. First of all, by all means, help someone else in need. ESPECIALLY if they are your "enemy". Who is our "enemy" after all? The first thing that comes to my mind is someone who hurt you.
This Scripture says that we will heap burning coals on their heads. This isn't a literal thing, and I wouldn't recommend you being nice to your enemy just to see them hurt. You know that old saying, "Kill them with kindness"? Yep, that's what you do.
Because in doing so, you are showing that you are the bigger person.
You are letting that person see that you can move on and be someone they aren't.
Peter asked Jesus in Matthew 18 how many times we are supposed to forgive someone who hurt us. In verse 22, Jesus said, "I tell you, not seven times, but seventy times seven."
It is not easy to be nice to someone who has betrayed you. I can tell you that first hand. However, it is worth it.
If we hold grudges against the people who hurt us, all we are doing is hurting ourselves. It is like us trying to poison someone, but we are actually drinking the poison. That person might not even know they hurt you, honestly. If they did hurt you on purpose, and they know it, holding a grudge is you giving them control over you. Why do that?
The last line says that the LORD will reward you. I'm not sure what that looks like exactly. I guess it really doesn't matter anyway. Anything from God is good. So, I'll take it. How about you?