Teddy is intoxicated and isn’t being reasonable. He seems to decide that if nobody is going to play along and let him pretend to be a good guy, he’s going to point fingers at other people to deflect attention from himself (such as by suggesting that Dad tried to kill Eli and August). This is insulting to Mum, however, so she starts to verbally fight back. But for Teddy, it’s essential that he assert his power over Mum, Eli, and August, simply because they refused to pretend that Teddy is good.