Complex sentence: 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.
Split into simpler sentences: Believing that Robin has been killed in the chaos, Dick Grayson beats Joker to death, although Batman revives his foe to keep Grayson from being a murderer. Nevertheless, the Joker's actions succeed in making a member of the Bat-family break their rule against killing.