6 Then the LORD said to Cain, "Why are you angry? And why has your countenance fallen?
7 "If you do well, will not your countenance be lifted up? And if you do not do well, sin is crouching at the door; and its desire is for you, but you must master it."
8 Cain told Abel his brother. And it came about when they were in the field, that Cain rose up against Abel his brother and killed him.
God said anger must be mastered. Here is a suggestion:
1. Ask God to help you identify the source of your anger.
2. Ask God to help you properly address the person or persons you are angry with (writing a letter explaining what happened and how it made you feel is very helpful).
Include in the letter the fact that you talked to God about what they did and that you forgive them.
Stamp the letter and drop it in the mail if possible.
If the person is deceased, do a ceremony and tear the letter up at the end, and throw it in the trash (this helps bring closure).
3. Ask God to guard your heart so that when you see that person again you will be nice to them. You may or may not get close, but you do have to forgive and be loving (Rom. 12:17-21).
Christians, put anger in check by following the above process.
If you are in a dangerous situation, or a criminal situation, etc., immediately get professional help.