SPOILER ALERT: The following article contains major spoilers about Harry Potter and the Cursed Child, and is meant for those who have either seen the play or read the script book. If that's not you, read on at your own risk!
One of the more twisted aspects about Harry Potter and the Cursed Child is the way it plays with timelines, delivering alternate realities that exist due to certain changes being made to the past. The result is the revelation of some very interesting things we might not have known about certain characters. We've already discussed the revelation about Snape, as well as the key question the play answers about Ron and Hermione's relationship. Now let's talk about that time Cedric Diggory became a Death Eater and killed Neville Longbottom, ensuring one of the darkest timelines ever.
Bellatrix and Voldemort having a Voldebaby is probably the most WTF reveal of Harry Potter and the Cursed Child, however Cedric's dark turn, and Neville's premature demise in the darkest timeline is a fair runner-up, especially as it paves the road for Harry Potter's death and Voldemort's rise to power. This timeline brings a couple of things to light...
The first, is that Cedric Diggory had a dark side. Don't we all? I will preface the rest of this by saying that I don't think Cursed Child is trying to make an argument that Cedric was always going to be a future Death Eater, had he lived long enough for it to happen. I think that, had things gone well for him, and he went on to win the Tri-Wizard Tournament and not died, he probably would've gone on to live a decent life doing whatever the nice, popular guy in wizard high school ends up doing. Maybe find some mid-level job at the Ministry of Magic, get married, have some kids, etc. What Cursed Child does is present a scenario in which things do not go well for Cedric, and demonstrates just how badly a person's life can spiral from one specific event.
To recap the key event in question, in an effort to save Cedric's life, Scorpius and Albus go back in time and try to change the Triwizard tournament. Part of this involves humiliating him at the second task, using an engorgement charm on him so he blows up like a balloon in front of the whole school.
Cursed Child's darkest timeline drastically alters Cedric's adolescence, which sets him on a different path. Being utterly humiliated during the second task changes everything for him, and in the process, proves that even the nicest guy can have a dark side if pushed hard enough. At the very least, Cursed Child proves that Cedric Diggory was more fragile than we might have guessed, if humiliation could lead him to eventually become a Voldemort supporter and an eventual murderer. It seems like such a drastic turn, but if we consider that he was revered by his peers and adored by his parents prior to being humiliated, the loss of respect and popularity could have had a huge impact on his self-esteem, making him particularly vulnerable to Death Eater recruitment.
On the other side of this, while Cedric's fall from grace changes the entire course of his life, it also creates a scenario that proves that Neville Longbottom's contribution to the Battle of Hogwarts was a massive game-changer. Ok, we kind of knew that part already, but it's still worth examining.
In Harry Potter and the Deathly Hallows, on his death-walk to the dark forest, Harry comes across Neville and asks him to kill Nagini. Neville makes sure to do this, putting his life on the line to get it done. And that eliminates the last remaining horcrux, ensuring that when Harry kills Voldemort, it's for good. It's not as if that magnificent moment were ever diminished in J.K. Rowling's book. Still, Cursed Child proves just how crucial Neville was to that battle, by taking him out of the equation and showing us what would've happened had he not been alive to kill Nagini. Neville's death at the hand (or wand) of Cedric means Nagini doesn't die, which somehow leads to Harry dying and Voldemort winning the Battle of Hogwarts. The specifics on exactly how all of this went down aren't given, but the drastically altered course of events provide evidence that Neville was a vital part in the success of that battle. Without him, the whole thing falls apart, and Voldemort rises to power.
We don't know the exact circumstances surrounding Neville's murder, but the fact that he might've been a target isn't entirely surprising, especially if we take into account his rebellious ways in Deathly Hallows. As for Cedric, at first, it was hard to really make sense of Cedric being so humiliated that he'd become a Death Eater. It seems like such a leap, and being a play, Cursed Child's structure doesn't really allow for any in depth explanation of how exactly he went from A to B, only that it happens. Perhaps J.K. Rowling will find a way to fill in those blanks for us, to at least give us a sense of how he turned down that dark path.
In the meantime, the connection between Cedric and Neville becomes tragically fitting when you put the two characters side by side and see just how opposite they are from one another in many respects. Cedric had a pretty wonderful life. He's attractive, incredibly popular, Seeker on his house Quidditch team, strong enough of a wizard to get into the TriWizard tournament, and his family seemed relatively happy and whole. Maybe his dad was a bit proud, which comes with its own pressures, but they seemed pretty close. Neville, on the other hand, is picked on at school, he's forgetful, bossed around by his Gran and altogether rather timid. His closest friends can likely be found in Professor Sprout's greenhouse. Not to mention, his parents are in St. Mungo's, having been tortured to insanity by the Lestrange.
In one reality, one of these characters dies, and the other becomes a hero. In the other reality, one of these characters becomes a villain, and the other dies. It's oddly symmetrical and tragic either way you look at it.
While it would've been nice to see Neville (aka Professor Longbottom) pop up at some point in Harry Potter and the Cursed Child, I think fans can appreciate that his representation in the play demonstrates the massiveness of his contribution to the Battle of Hogwarts and the reality that follows. Meanwhile, we also get a fascinating glimpse of how things might have turned out for Cedric, under certain circumstances. It's a mark of the complexity of Rowling's characters that such a scenario can even be explored. And even if that timeline didn't stick, we can never really un-know that side of Cedric Diggory.