We don't have to be 'nice' all the time. Some people are very shocked that such feelings are even residing inside themselves. Just acknowledging that they are there and speaking it clearly within a group setting can be enough to help relieve any deep internal tensions. As Freud
noted early on in the 20th century, it takes alot of energy to be civilized.
In the world of Breathwork all feelings are welcomed, all energies are welcomed and worked through either through the breath itself or some integration work at the end of a session.