MikeH

Question: Why would Harry say Voldemort killed Cedric? Obviously it was Wormtail.

MikeH

Answer: Wormtail was acting on Voldemort's order. Voldemort ordered Pettrigrew (Wormtail) to, "kill the spare," meaning Cedric.

raywest

With Voldemort's wand even.

lionhead

Answer: Peter Pettigrew was believed to be dead by the entire wizarding world. The only witnesses are Lupin (a werewolf), Sirius, who is still on the run, Harry, Ron and Hermione. No one believed him about Voldemort returning, as they thought he was only seeking attention. Its safe to assume everyone would also think the same about Pettigrew being alive.