User blog comment:Abranon/Why didn't Regulus Black do more to contribute to Voldemort's Death?/@comment-306173-20120122130318

I don't know what books have you been reading. Regulus was killed by the Inferi in the cave, he did not kill himself. He couldn't just "decide" to live long enough to destroy the Horcrux. He had to drink the emerald-green potion so that they could switch the real Horcrux with the fake locket, and by doing so, he had to drink the water from the lake, thus summoning the Inferi. Only Kreacher could escape the cave (Regulus couldn't Disapparate because of the charms around the cave, but Kreacher could leave using his house-elf magic), so Regulus ordered Kreacher to try and destroy the Horcrux, while he was left to die. Regulus had not destroyed the Horcrux, so, no, Voldemort was never mortal at the time. Any attempt to kill Voldemort would be foolish, would mean and certain death. Voldemort only became mortal moments before his death on 2 May, 1998, when Neville killed his last Horcrux, Nagini.