It's a general point but it was in response to killing Aradia specifically, which is an action that was (in part) goaded on by Scratch's meddling. The only difference between Doc convincing Vriska to kill Aradia and Doc engineering the rest of Troll society is that Doc had direct conversations with Vriska. The argument Doc Scratch made, and the reason Vriska ultimately does it, is that the action was pre-determined. Vriska isn't any more or less responsible for her inevitable actions just because Scratch told her about it in advance.

As for creating Bec Noir, Vriska knew Bec had to be created, but she didn't know she was the one who had to be responsible. She WANTED to be responsible, to make herself feel important, which is kind of the same logic Aranea used to justify her own meddling leading up to Game Over