Anger isn't an emotion. It's a behavioral expression of unresolved feelings. It may stem from underlying feelings of guilt, fear, hurt or rejection. It's It's reaction when person feels vulnerable and insecure. Anger keeps simmering and surfacing, if the real emotion is unaddressed.
You are welcome to add your thoughts.
That's an interesting premise, and I fully agree that some underlying trauma, conflict can make one angrier and predisposed to anger but this in of itself does not mean anger is not an emotion. Anger as emotion is well-recognised and along with several other emotions even has a unique facial expression (see the work of Ekman). Anger is definitely an emotion but you are correct that anger can be made worse by trauma. A simple thought experiment can show that anger is somewhat innate. Imagine an individual loses a loved one due to someone drinking or driving, would we say their anger was due to past trauma or for some underlying issue with the drunk driver? No we would just accept the anger as a normal response to an injustice. (Ayesha made a similar point) Good question though and some interesting points.
But according to APA, anger is an emotion. It is a negative emotion. It can be good and bad depends on you. If you are how anger can be good. It can motivates you to find solutions to the problems normally you cannot solve.