He may have seemed like an easy character to kill off when we first read the books but clearly there was a bigger significance to Cedric Diggory in the Potter series.
Cedric Diggory is the kind of character that showed up for a moment then left and we didn’t think anything different of him. But then, in Harry Potter and the Cursed Child, we can see that he is the kind of character whose death clearly weighed on Harry for quite some time. But why is that?
To be honest, it’s probably because it is the first that Harry saw directly as a result of his supposed failure. To be fair to Harry, it wasn’t his fault. But when he looks at it, he sees himself bringing Cedric to his death.
Which just goes back into Harry’s guilt for living through all of the death surrounding Voldemort and the Battle of Hogwarts. So many died so that Harry Potter could live and Cedric was the beginning of that. Sure, his parents are there but he didn’t know them and didn’t really have that much of a hand in it. It was Voldemort taking a prophecy and misunderstanding his role in it.
So with Cedric, Harry saw his death as something that he knew about and couldn’t stop. He stood there and watched him die and so Voldemort’s daughter used that against Harry in a way that not many had foreseen. Harry struggling to understand his pain with Cedric? It is certainly interesting and it sucks that that’s what Harry continually has to deal with.