I don't understand how humans can do such things. They were friends, they went to bar together.

What relevance does it make to them if Brandon was a guy or a girl? Why so much hate? How come everything changes like that when they find out?

They got so deeply insulted by such detail and see that unacceptable. But they consider OK to rape her the whole night (not just twice as depicted on the movie) and then kill her, the girl and the other guy.

This show their character and their values.