I think people are focusing too much on the “message” behind the rules governing the Force and the dark side, and not enough on the fact that it’s just a fantasy magic system.
The dark side is only something that applies to people who actually use the Force. This means that it is a risk associated with that particular activity. People who do not use the Force, such as Han Solo, can be as morally gray as they want without any risk whatsoever of falling to the dark side. So the message isn’t “anger is always bad”, it’s “you need to be tremendously careful and have tremendous self control when dealing with this mysterious fantasy power”.
The whole appeal of fantasy is that it allows us to explore the human condition from angles not possible in reality. And in the case of the Force, it allows writers to explore the idea of how people would respond to a situation in which they have access to this mysterious power that constantly puts them at risk of being consumed by their negative emotions. It’s not supposed to be a direct moral lesson for real life, because the dark side doesn’t exist in real life. You need to stop thinking about real life and just immerse yourself in the fantasy.