Regulus is indirectly killed by a series of magical enchantments set up by Voldemort. Not by his direct hand, no, but by a setup to protect the horcrux. Regulus was selfless enough to retrieve the horcrux because he was horrified by what Voldy was planning...immortality. Whatever they do in the film is for the benefit of making a movie flow easily for a wide audience, not just a bunch of HP addicted fans. I dont like changes like this, I am a stickler for detail, but I do understand some of them. Besides that, the fact remains, they wil continue to make the movie as they see fit and w/ JK's blessing, whatever we say doesnt change a thing!!!!