Six conditions that make anger sinful:
1. When, to favor a resentment or feud, we imagine an injury done to us.
2. When an injury done to us becomes, in our minds, greater than it really is.
3. When, without real injury, we feel resentment on account of pain or inconvenience.
4. When indignation rises too high, and overwhelms our ability to restrain.
5. When we gratify resentments by causing pain or harm out of revenge.
6. When we are so perplexed and angry at sin in our own lives that we readily project anger at the sin we find in others.
Robert Speer