Believing that Robin has been killed in the chaos, Dick Grayson beats the Joker to death (although Batman revives his foe to keep Grayson from being a murderer) and the character succeeds in making a member of the Bat-family break their rule against killing.
Robin has been killed in the chaos. This is what Dick Grayson is believing. Dick Grayson beats the Joker to death. The character succeeds in making a member of the Bat. Family break their rule against. Family is killing.