As shown by the fact God Himself can get very angry, anger is sometimes justified. However, anger is more often than not both irrational, and a gateway to wickedness. It is, unfortunately, an ever-burning fuel for itself. Anger produces more anger. Thus, we must learn to control it.
When someone sins against us, we might be tempted to react sinfully. "This person cut me off, of course I'm going to flip the bird!" What? He sinned against you, so you sin against him? Since when have two wrongs made a right? Not biblically! Far better to calm yourself and respond appropriately. When you are angry, Satan can capitalise on that. He will whisper many evil ideas in your ear, and if you're not thinking straight, you may well entertain them.
But what if you're thinking in a Godly way? This person, in sinning against me, has sinned against God. As David said having taken Bathsheba and killed Uriah, "Against You, and you only, have I sinned" (Psalm 51:4). Well now it's a whole different ball game, isn't it? I don't need to take revenge against this person, because if I do, I can almost guarantee I'm going to do it wrong. I may be too lenient, giving them less than they deserve, but far more likely I will be too severe, not only sinning against God, but ultimately deserving more wrath myself.
But God is not like this. Aside from the fact He is able to justly punish the sinner, He doesn't actually want to punish the sinner. Tell me, my brethren, who punched your ticket to Heaven? You were God's enemy! You sinned against Him, you stored up wrath for yourself, and the Bible says God is angry with the wicked every day (Psalm 7:11). And so where would you be if God could not control His anger? Where, then, does God intend us to place others when they make us angry?
When someone cuts you off, you do not need to flip the bird. Flipping the bird is a sign of anger, and is not only expected, but gives a feeling of having been wronged. But what about a thumbs down? Well now I just feel bad for cutting you off... That hurts, man. Perhaps I should be a better driver in the future. You see, then, how that thumbs down says more in one gesture than you could possibly say with a thousand words. The anger has been removed from the situation, and now some deep thinking can begin. The result? Correction.
And this is where we are supposed to go. Jesus tells us that when we are wronged, we're not supposed to react in anger, but follow a process to "win" our brother. First, tell their sin to them alone. If they will not listen, bring witnesses. If they still will not listen, tell the Church (Matthew 18:15-17). The goal each time: Reconciliation.
If we are to imitate Christ, anger should be almost foreign to us. When it raises its ugly head, it should be justified, short lived, and certainly not in control. God Himself is slow to anger, and easy to appease. So also must we walk with self control and a sound mind, ensuring that when the devil tries to get his foot in the door, he limps away with a broken ankle. May justice prevail in our lives, and may mercy prevail over wrath.