Um pouco de JKR sobre o assunto:
Hayleyhaha: Why did regulus have a change of heart
J.K. Rowling: He was not prepared for the reality of life as a Death Eater. It was Voldemort’s attempted murder of Kreacher that really turned him.
No que diz respeito à tentativa de morte, o Wiki de Harry Potter fala sobre isso.
In 1979, when Lord Voldemort told Regulus Black that he needed a house-elf, Regulus volunteered Kreacher, saying it would be an honour to serve the Dark Lord. Voldemort took Kreacher to the Horcrux cave and forced him to drink the potion in the basin so that Voldemort could place Salazar Slytherin's Locket in the basin. The potion, (the same one Dumbledore drank) caused Kreacher to have nightmarish delusions, intense stomach pains and made him dehydrated. Voldemort then left, leaving Kreacher to drink from the lake (as any water conjured evapourates) and die at the hands of the Inferi. However, Voldemort did not remember (or did not realize or thought that the cave's magic was too strong for anyone or anything to disapparate out of) that house-elves have their own special type of magic and Kreacher was able to Disapparate from the cave as Regulus told him to come home when he finished his task.