I'll join. (I hope this is okay, but I'm going to argue a little with what Alo said.)
It wasn't necessary. But that's just it. Voldemort didn't kill because it was necessary. In fact, Cedric was one of his more reasoned murders. Now that I think about it, Voldemort didn't even kill Cedric! Wormtail did! Voldemort couldn't have anyone know he was back. So he killed Cedric. He meant to kill Harry, but he couldn't. JKR wasn't doing what she wanted to do. She wrote exactly what Voldemort would have done.