Before anyone gets all upset at the topic, let me say it's not a sin to be angry. But, that being said, it CAN be a sin, depending on how you react to it. AND if you react by lashing out, it can be a sin not only against others but also against yourself.
First: Not a sin. Being angry is an emotion, one that everyone has at one time or another from babyhood to death. Anger can be positive; it can lead us to become involved in something that leads to others being helped. Being angry at sexual exploitation of children would be one example of it not being a sin, especially it it leads you to become involved in one of the organizations which free or provide help those enslaved by or who have been enslaved or preyed upon by sadistic exploiters. Anger can provide the impetus to get involved.
Second: Emotions are not in themselves sin. However, when an emotion of any kind leads you to harm others mentally or physically, it is sin. Let's call a spade a spade here. Last weekend I became very frustrated at a computer program which misbehaved. I got online with tech support and ranted at them for awhile. In the middle of this, the phone rang--a political call. I saw the caller ID, picked up the phone, and before they could say hello, I said, "I don't want to make a donation. Goodbye." And I hung up.
Immediately the same outfit called back, and a young lady's voice said, "Wait! We don't want a donation!"
I said, "So this is a survey? Well, I don't want to make a donation, I don't want to answer a survey, and I don't want to talk to you. Anything else?" (Keep in mind, my tone of voice was not exactly friendly.)
The young lady on the phone hesitated, then said, "No, ma'am." Politely. Apologetically.
I hung up on her again. And I'm just guessing, but I don't think it would have done any good for me to tell her in that same tone of voice that Jesus loved her and so did I.
Third: Did I feel better? No. In fact, the anger wrapped around my heart and squeezed. I could physically feel it. I sinned not only against a young lady who was simply doing her job, perhaps a job that kept a roof over her head and food in her children's stomachs, but I sinned against myself.
I can understand now how people who unleash their anger have heart attacks and strokes.
I sinned, Lord. Please forgive me, and please forgive those who read this and see themselves in the reading. Thank You that You don't unleash that kind of anger on us. Thank You for forgiving, loving, and leading us into serene pastures with calm water. I love You, Lord.