
After I posted that I remembered that Myrtle was killed by the basilisk. not by Tom. she heard him opening the chamber but he wasn't the one to kill her. Sorry
But but ... It appears that you might get "credit" for killing somebody if it's done by your agent. It's not clear, but I think we're meant to think that Myrtle's killing enabled Tom to make the diary into his first horcrux. And Frank Bryce was killed by Nagini, and Dumbledore was sure that Nagini is a horcrux. Would Jo have kept associating significant deaths with horcruxes, and then beg off by saying it was a different killing that was responsible? Some poor unrecorded soul died for the diary, and another unrecorded soul for the snake? That just doesn't seem very neat to me.
Lily and James are the only ones we know were killed directly by Voldemort. We only have his word for it that he killed Tom Riddle Sr and his grandparents up close and personal. That's why I tend to believe it's killings we know about that are the important ones, even if the act is only performed on Lord V's orders.
Of course I'm almost certainly wrong, again!