In plain words
The reason: they used anger with a selfless feeling. In order to further sacred works, to curb a little man's evil conduct, to wipe out Ajñāna and Ahaṅkāra, and to protect others, anger has to be used on certain occasions. Durvāsa's anger did only good on every occasion. The reason: he used his anger selflessly. When we use anger without leaving Satya, without breaking Dharma, on a selfless path, it travels on a sacred path.
The moment a saṅkalpa is born, we must ask whether it is good or bad
In just the same way, peace, forbearance and compassion at certain times go against Dharma. If a king skilled in governing were to adopt these three, kindness, forbearance and compassion, his very rule would come to a standstill. When he goes on forgiving all who have done wrong, showing compassion to all the wicked and looking on them with kindness, the order of his rule goes wholly off the road. As important as the goad is to an elephant, as much its support, as important as the bridle is to a horse, just so important is punishment for wrongdoing. Without that very punishment there is no protection at all. That is why we may hold that, in humanness, some saṅkalpas are noble. We may believe that some are bad. The effects of good and bad rest on the mind. So when the mind takes up pure and selfless feelings, everything turns out good. That is why we have to enquire into the effects of saṅkalpas with great subtlety. To believe in the forms of these saṅkalpas on the physical and natural plane is very hard. When, with no bad feeling at all in the heart, we let our saṅkalpas arise purely and nourish good saṅkalpas such as ‘Lokāssamastā sukhinobhavantu’, that all must be well, everything goes well.