Board Thread:Questions and Answers/@comment-33725506-20171122150003/@comment-26907577-20190801134134

Sans boi2 wrote: I know you can fight him but how like kill everyone or pasfist? You fight Monster Kid in a Genocide Route, immediately prior to fighting Undyne (as the Undying if you choose to fight MK.)

[Genocide spoilers follow]

Attacking Monster Kid causes Undyne to appear and take the blow. Monster Kid runs and Undyne uses determination to reform herself into the Undying, who you then fight.

Monster Kid does not die in any route of Undertale.