Sing a Song of Murder

Large print for those whose eyes are as bad as mine
Allen Barker hit the big time. All of a sudden everything he did turned to gold or platinum. He had found his dream. He was a superstar.
Then his girlfriends started getting murdered. One, those things happen. Two, a sad thing. Was he jinxed? Would there be a third? That would be a long way over mere coincidence.
Who would be doing this? Why?