cluelessosity
~In Progress~
He was murdered by Voldemort. Or on Voldemort's orders, more likely. I doubt Regulus was ever important enough to be killed by Voldemort in person. From what I found out after he died, he got in so far, then panicked about what he was being asked to do and tried to back out. Well, you don't just hand in your resignation to Voldemort. It's a lifetime of service or death.
My brother also had himself a friend. She was a beautiful woman really. I liked her myself. But after Regulus died, she ran away with Alice and Frank, they came back tortured into insanity. My brother was everything to her as she was to him. Too bad, really, That he died the day before their wedding.