where his civilizations, including our own

Do you mean the bit where Sirius says Regulus was killed by Voldemort himself? Because if so, he then goes on to say: "Or on Voldemort's orders, more likely; I doubt Regulus was ever important enough to be killed by Voldemort in person." Which supports your idea here that Cedric wasn't considered important enough for Voldemort to kill himself (though there's some inconsistency here, because apparently Frank Bryce was).