The expression of the wrathful deities, the protectors, is not an expression of angry, but to show an impatience of wanting to liberate all the sentient beings as fast as possible. A little bit like a father or a mother loves their children so much and they want them to be better and better. In fact, if you really want to help other people, nobody can really help with a big smile. You will always have some impatient act. That is the act of all the protectors. Therefore, the wrathful expression seems a little bit ugly but has such a great love and joy in it.
With Love and Respect from Kalu Rinpoche
Jun. 2018 @HongKong Teachings