Who really was driving when Myrtle was struck and killed? Can Nick be sure? Can we? If Nick insists that a person shouldn't criticize others, then why does it matter who killed her?

Daisy was driving the car that hit Myrtle. We know this because Gatsby had let Daisy drive the car and admitted to Nick what happened. I think, in this case, it does matter what happened. Somebody actually died as a result of Daisy's actions.