Obviously it was Wormtail. He's not doing a very good job proving he's not a liar.
share“Wormtail killed Cedric! Literally! It was Ron’s former pet rat who used the number one unforgivable spell on him! By the way, Voldemort is back and HE ordered Wormtail to kill him without giving it a second thought.”
That wouldn’t sound as dramatic or as sincere as it should have. Harry was telling the truth.
I get him wanting to get the point across that it was Voldemort responsible but he never said Wormtail killed Cedric on command. He only mentioned Voldemort.
I guess it wasn't relevant how he died but it's still weird he never told anyone exactly how it happened. There was a short scene when the kids ask Harry how Cedric died and he refused to elaborate. I don't see what's the big deal telling the kids the truth.