Have you ever been angry with someone without cause? Or to rephrase that, have you ever driven on a freeway before?
Many people, in the depths of their heart, have anger and hatred to such a degree that their true desire would be for the one they hate to be dead. But that is clearly forbidden in Scripture. 1 John 3:15 says, "Whoever hates his brother is a murderer, and you know that no murderer has eternal life abiding in him." The word used here for hate means "to habitually despise." It implies not just a transient emotion of the affections, but a deep-rooted loathing.
The Bible is not saying that it is always a sin to be angry. But the idea in 1 John 3:15 is that of deep-seated anger and continuing resentment. It is to habitually despise.
Some people hold grudges. They operate by the phrase, "Don't get mad. Get even." Or maybe someone has wronged you. Maybe they have taken advantage of you or slandered you, and you've thought, "I hate them. I wish they would just drop dead." But there is no place for thinking like this on the part of the believer. God says, "Vengeance is mine," (Heb. 10:30). You have to let it go and forgive that person, whether or not he or she deserves it.
You have been forgiven a great debt, and as a child of God, you must forgive others. If we know anything of what God has done for us, then we must extend the forgiveness we have received to others as well.