Yes. Sadly, in one of the alternate realities that resulted from Albus and Scorpius going back in time, Cedric Diggory did in fact become a Death Eater. This happened after Scorpius and Albus insulted Cedric to prevent him from participating in the Triwizard Tournament to eventually save his life. However, in this reality, Cedric went on to become a Death Eater and eventually killed Neville Longbottom, who in the real timeline killed Nagini, thereby destroying all the Horcruxes of Voldemort which helped Harry kill Voldemort and end the war.