I thought she said in an interview she was going to wait a year before starting on it....
Anyway, I don't know if anyone said this before, but did you realize Snape made the Unbreakable Vow?
' "And should it prove necessary . . . if it seems Draco will fail . . ." whispered Narcissa (Snape's hand twitched within hers, but he did not draw away}, "will you carry out the deed that the Dark Lord has ordered Draco to perform?"
There was a moment's silence. Bellatrix watched, her wand upon their clasped hands, her eyes wide.
"I will," said Snape. '
We already know now that Malfoy's task was to kill Dumbledore. He couldn't do it though, and Snape ended up killing him. Would this be the reason? Knowing Snape, he would rather let Dumbledore die that he himself.