What made Tom beat his mistress, Myrtle?

The group becomes exceedingly drunk at the party; as a result, Myrtle begins to grow garrulous and harsh. Shortly after Tom gives her a puppy as a gift, Myrtle begins chanting Daisy's name to irritate Tom. Tom tells her that she has no right to say Daisy's name; she continues taunting him, and he responds by breaking Myrtle's nose.

Tom is both a bully and a hypocrite: he carries on a highly public affair, but feels compelled to beat his mistress in order to keep her in her place.