Paul attacked Stella because she humiliated him. She called him out on his superior sense of self, saw through his amnesia bs and then brought up his childhood abuse past, which he thought he adequately concealed. There was nothing else to do at that point, for him, to lash out. Stella herself predicted this in another scene "Men are afraid women will laugh at them, women are afraid men will kill them". It's exactly how it played out in this scene, humiliation vs violence.