In Vajrayana practice, guru devotion is very important for us to gain attainment. All Mahasiddhas such as Naropa, Milarepa gained enlightenment due to their strong guru devotion. Tantra practice is a very fast way to gain attainment and it is very important we do it in accordance with the instruction. If we cannot follow our guru's instruction before receiving tantra practice, how are we sure we will be able to follow the Tantra practice instruction?
Our guru has a mind of a Buddha but he is very kind to manifest in human form so he can guide us. Since our guru is a Buddha, he will never harm us. When our guru manifest anger or he is displeased with us, it is because he is helping us to improve and clear our deluded mind. Only a real guru will be kind enough to show us his wrathfulness because he wants to help us to clear our obstacle or negative karma quickly.
If we feel angry when our guru uses the wrathful method on us, it will not help us to grow or gain realisation, it will only create the cause for us to be further away from our guru and Dharma. If we contemplate and regret we will see our mind changes for the better. We have to always remember, it takes a lot of merits to meet a qualified guru. When we have found one, never to abandon our guru. Whatever hardship we are going through in our spiritual practice is nothing compared to losing our guru.