Alternative Character Interpretation: Rocky Sullivan. In the end, did he truly freak out at being sent to the electric chair, or did he act scared just to let all the kids know he was a bad example to follow?
Deconstructed. He makes being a gangster look so awesome, the Dead End Kids want to grow up to be just like him...which would lead to spending the rest of their lives in and out of reform school and prison and an early death in the electric chair.
Evil is Cool: In fact a main plot point of the movie is Connolly trying to convince the Dead End Kids that what Sullivan does isn't cool, without much success, until perhaps at the end, after Sullivan went to the electric chair screaming. As to whether thats cool, it's left extremely unclear if he did so in order to convince the Kids, or if he actually freaked out.
Tear Jerker: The ending. Rocky ends up screaming and begging on his way to the electric chair, leaving it ambiguous if he is faking to destroy his image to the Dead End kids or if he genuinely broke down. Father Jerry ends the film by asking the shocked kids to join him in "saying a prayer for a boy who couldn't run as fast as me."