User blog comment:Abranon/Why didn't Regulus Black do more to contribute to Voldemort's Death?/@comment-68.226.202.135-20120122141711

Regulus actually did do lots to contribute to Voldemort's death. When he told Kreacher to destroy the Horcrux, he believed that it was the only one. Therefore, he was forcing Voldemort to be mortal, and allowing someone from the "good side" to finish him off for good. About the potion, it forced one to relive one's darkest moments; Regulus was in desperate need of water and did not want to waste time by Side-Along Apparating with Kreacher. There was no way for him to leave the cave, except by drinking the water in the lake and being dragged below the surface by the Inferi.