Regulas

A Death Eater could have found them coming back, and killed Regulas a few days after his seventeenth birthday, since he only managed to live a few days after deserting the Death Eaters.

Willa Rubin: Six Speculations About the Seventh Harry Potter 2008

Then when they got across, Regulas and Snape could have replaced the locket with it's decoy, and the note written inside of it would have already been pre-written.

Willa Rubin: Six Speculations About the Seventh Harry Potter 2008

Assuming that Regulas and Snape were in Voldemort's inner circle around the same time, there's a small chance that Regulas found out about the horcruxes, but through investigation, and then told Snape -- Voldemort's still not going to go around telling his "friends" that he is creating horcruxes.

Willa Rubin: Six Speculations About the Seventh Harry Potter 2008

Regulas would have had a change of heart, and wanted to stop Voldemort -- but not directly.

Willa Rubin: Six Speculations About the Seventh Harry Potter 2008

Snape would qualify to cross the lake with Regulas.

Willa Rubin: Six Speculations About the Seventh Harry Potter 2008

If Regulas was in Voldemort's inner circle, he could have known about the diary, which went into the hands of Lucius around the time of Regulas's death.

Willa Rubin: Six Speculations About the Seventh Harry Potter 2008

The Death Eaters might have thought Regulas was older -- so they thought he turned eighteen when he actually turned seventeen.

Willa Rubin: Six Speculations About the Seventh Harry Potter 2008

But since Voldemort did not suspect that Regulas knew about the horcruxes, Voldemort still might have asked Regulas and Snape to hide the locket in the cave.

Willa Rubin: Six Speculations About the Seventh Harry Potter 2008

It might have been days before Regulas turned seventeen, but he still qualified as an underage wizard.

Willa Rubin: Six Speculations About the Seventh Harry Potter 2008

Sirius said to Harry that Regulas, like his parents, had been involved with all the pure-blood mania in OotP.

Willa Rubin: Six Speculations About the Seventh Harry Potter 2008