Ephesians 4:26 says, “Be angry, and do not sin: do not let the sun go down on your wrath,” One of the problems with this verse is we have trouble naming anger for what it is. We call it frustration, or disappointment, or upset, or righteous indignation, or any one of an infinite number of names which allow us to deny we are feeling angry. A good test of anger is, Is it someone or something else’s fault? If so it is likely just anger.
So what is sin with anger? When we make someone else pay for it, often in subtle ways. We refuse to let the other person know the mistake you see them making, or we withdraw from them and make ourselves unavailable when it would be appropriate to be available, or we gossip about them, or we get someone else to do something negative toward them, or we get sarcastic toward them, There are many other ways we sin with anger.
The second part of the verse is written only to the angry person, not to the person we are angry with. It is telling us YOU deal with YOUR anger quickly, or you will sin against someone.