I heard that the killer approached Dimebag saying that he broke up Pantera as well. Whilst what Todd says is true, there was a period when both Dimebag and Vinnie were saying they weren't certain of the future of Pantera, while Phil was saying he wasn't interested in reuniting, so it was him that made the first move. Of course they are going to form a new band when they have been told that their current one is not going to continue or reform.
I agree with Spawn, it was very unexpected that it was Dimebag. I wouldn't have been half as surprised if it had have happened to Phil, and it is a pretty scummy thing to say, but I wish it had have been Phil.