Why did the Volturi want Victoria to succeed in The Short Second Life of Bree Tanner?

The Volturi feel threatened by the Cullens. They are the largest coven excluding them, and because the Cullens are talented in their special ways, the Volturi feel threatened. If Victoria succeeded in killing Bella, that would in turn kill Edward, as he would provoke the Volturi.

From there, the entire Cullen family would spiral and things would get so bad, some of them would go to provoke the Volturi.

If Bella was killed, and the Cullen family lessened in numbers, the Volturi would be much safter of being overtaken, even if the Cullen's don't and didn't plan on it.