In short, anger and wrath are seen as emanations of skillful means in Buddhist magick because they reflect the nature of what's awakened as clear direction in the mind and heart when it happens. You see, sadness and confusion, they represent dejected states of being....a way of being adrift, rather than being at the forefront of destiny and its functions.
Anger, wrath, rage, once awakened, become the modes and means of transformation and moving the vehicle of your world through to whatever the next stage of your existence will be...the trick is that so many people want to use this rage ahead-of-schedule, putting the cart before the horse because they're mad at the pattern rather than being committed to understanding AND changing it.
Instead, in the West & in the uninitiated mind, anger is an arrow that flies without thought, or a bullet fired without consideration. It is a weapon and a poison both...
To the skilled Vajrayanist, however, anger is a tool and a means of management; a way of transforming a situation through intentional use of that rage. Consider: if you could either demolish a person or a pattern, which would you choose? The emanation of your unskilled, unrefined subconscious made manifest as the individual you're having that painful dialogue with, or the actual machinations within you that create that continual, cyclical existence through circumstance?
That's the difference between skillful means and poisonous wrath...and this is what we learn from these wrathful, angry manifestations of the Buddhas.
Hayagriva, Dakiniten, Manjushri/Monju Bosatsu, Vajrakumara, Hachiman, and others each have means of making themselves delightfully present, and its your awakened mind directing these activities. If you do this blindly, you will destroy your life - these teachers help you to see, to understand, and to act with _intention_ in this world that the circumstances might change.
...and this is what I Tea-ch. ;)