I too see anger (and all other emotions we experience) as an evolutionary adaptation. It is more a neurological fact than a metaphysical matter woven into the fabric of reality, in the manner that philosophers tend to discuss it.

In this case, anger is a loss response. Sadness is also a loss response but it looks back in time and works to reduce one's activity level, while anger is forward looking and tends to drive one towards more action. (Unrelated, but fear is also a loss response, but a response to a potential loss).

Anger also generally (but not always) requires that the one experiencing anger recognizes the perceived cause of anger as an agent with free will. We don't feel angry, for example, when we get caught in the rain (unless it is at ourselves for not bringing an umbrella). However, we have seen animals (such as chimpanzees and dogs) on occasion expressing what look like anger at inanimate objects that have hurt them.

Perhaps there are other intelligent species out there with a different evolutionary path that has a larger number of primary emotions. And perhaps there are others who only possess one emotion in place of both anger and sadness. It is a subject worth examining from an evolutionary and neurological angle, rather than philosophical.

A phenomenon like road rage can be explained in a similar way: since the interaction in that situation is brief and unlikely to be repeated, it makes sense on some level for the response for the perceived slight to be disproportionately intense (“condensing” the lifetime of revenge in a single outburst).

Seeking revenge, with premeditation and planning is not unique to humans. A lot of animals also hold grudges against others and humans. So there must be some sort of evolutionary advantage that might not be too obvious.

Game theoretically, seeking lose-lose as revenge is not rational. But credibly precomitting "burning your ships" to revenge is rational.

A gene for anger is a pre commitment.