The murder was most likely spur of the moment- who could he have told in the five seconds before Snape blasted him off the tower? Besides that, Dumbledore can only do so much for the order- direct other people where to go, so to speak. Snape, under the influence of that Unbreakable Vow, would be more able to change things what with his connection to Tom, which would be strengthened by taking out one of his biggest rivals.
Plus you're not looking at the whole situation- Snape could've killed Ginny, Flitwick, Hermione, Harry- everyone he came in contact with, really. His cover was going to be blown anyway, what did it matter if he took out a few mudbloods and blood traitors?