Chazal compare someone who gets angry to an idol worshiper – because they lose control and no longer can think rationally. That sort of anger is never good or healthy.
Yet anger of a different sort can be good, sometimes, I think. Say someone gets angry at an injustice, and it propels him to take action to correct that injustice. But that is not the type of anger where one loses control, I am not sure it would even qualify as the basic definition of anger.
Slow to anger means it takes a lot to get angry, the person does not easily get angry.
