"He that is slow to anger is better than the mighty."
The first five Reiki principles by the founder, Usui, state "Just for today, don't get angry." Anger, though negative, helps us focus attention, gets the adrenaline going and helps us charge resolutely towards a goal.
Reiki, too, is fundamentally about pursuing a goal and trying to focus attention, however minus the ego. So, take a pause when you get angry and ask yourself a few questions; why you are angry, what does this anger have to do with anything, why you're wasting your time on it, etc. These questions will help you justify your actions and help you act in a more balanced way. This practice helps us let go of unnecessary, harsh anger.
Going by what Buddha said, "Holding on to anger is like grasping a hot coal with the intent of throwing it at someone else; you are the one who gets burned." The picture that comes to your mind is you tossing the hot burning coal persistently without dropping it to harm the other person. And while you do so, you only burn your hands as the person runs off. That's what anger does to you; it harms you in ways you cannot imagine.
If you let go of your ego for a while and keep questions like, "how dare he did that to me", "I can't take this", "who is he to hurt me" and replace these with logical reasoning, we will be living a fuller, happier life. The problems of the world like war and poverty will disappear. Of course, the world cannot change in a day. But with one person at a time, we can surely hope for a day when we won't have these issues. We can focus on more positive things like growth and peace.
Follow the founder's wisdom and stop anger when it first comes up.
Just for today, don't get angry.