A bit from JKR on the subject:
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.
As far as the death attempt, the Harry Potter Wiki talks about it.
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.
scifi.stackexchange.com/users/98/PearsonArtPhoto
From the books, the way Kreacher tells it, Regulus turns anti-Voldemort when Kreacher tells him the tale of what Voldemort did in the cave.
So that leaves us a combination of two black deeds that might have turned him: The knowledge that Voldemort made a Horcrux, which is very bad ass even for a Dark Wizard.
The knowledge that Voldemort was cruel to Kreacher.
According to Kreacher, he and Regulus were somewhat close.
I think it was mostly the first that horrified him and made him a turncoat, but the second can't exactly be ruled out.
(I wouldn't try to kill my PM because he killed my favourite pet, but I suppose some people might).
scifi.stackexchange.com/users/947/apoorv020
For many death eaters, the Dark Lord's persuit was a big hat that covers a wide range of causes: many did follow the same line as the Dark Lord's, ie subjugation of the Muggles; but some they simply wanted to live an open public life, free of the secrecy law, and all its trappings.
Regulus seemed fall in this latter category, as illustrated by JKR's writing in the book by Kreacher's telling of his young master.
He saw the Dark Lord simply as someone who could bring out this change.
Now he was a proud young man, proud of his heritage, not only his magic ability, but also his long-line Anglo-Norman aristocracy background (Motto: TOUJOURS PUR).
He was very close to Kreacher, a house-elf.
Serius might have only seen Kreacher as a servant, hellbent on hatred to Mudblood, but I suspect Regulus saw him as part of the family, a member of his magic world:
British Aristocracy's complex and somewhat paradoxical attitude of their family servants, lower social ranking, living downstairs, but still a family, and deemed family member's affection and protection.
With the Dark Lord's disposal of Kreacher like some ragdoll, I suspect Regulus saw through him, someone cruel not only to Muggles, but would inflict destruction to his fellow wizardry without hesitation.
At that point, Regulus had had enough, and decided to switch side.
scifi.stackexchange.com/users/119013/lukayl
I think maybe he just didn't realize what it really meant to be truly completely evil, as was required to be a death eater.
He maybe just didn't realize that the tales and rumors were actually 100 percent true, or worse.
And then he got to witness it firsthand, and decided it was enough.
He got disgusted by it, like when you watch happy tree friends.
YOU put on the video, but there comes a point when your like: Ew.
This is too disgusting.
I'm gonna turn this video off, throw up a little, and google pictures of puppys.
scifi.stackexchange.com/users/16365/Cecilia Colley
Regulus had just enough good in him to realize how big a mistake he made in joining the Death Eaters.
He saw Voldemort's capacity for evil and decided to fight against it, even if it meant sacrificing himself to the Inferi.
It looks to me like he and Sirius were more alike than perhaps either of them knew.
scifi.stackexchange.com/users/20359/Arachno-Sapien
Regulus Black was a Death Eater, but he was also the infamous R.A.B., who was the only person besides Dumbledore to discover the Horcrux's, and to hunt them down (Or at least, one of them). So, why did he have such a change of heart, to trying to kill Voldemort?
Why did Regulus Black change his mind?
scifi.stackexchange.com
<harry-potter>
scifi.stackexchange.com/questions/4704
scifi.stackexchange.com/users/98/PearsonArtPhoto
It was Voldemort’s attempted murder of Kreacher that really  turned him.  With the Dark Lord's disposal of Kreacher, Regulus saw through him, someone cruel not only to Muggles, but who would inflict harm on his fellow wizards without hesitation.  S3: It was Voldemort’s attempted murder of Kreacher that turned him away from becoming a death eater.   
S3: It was Voldemort’s attempted murder of Kreacher that turned him away from becoming a death eater.   