Meredith calling for Annulment had nothing to do with Anders when it comes down to it. Her decision to Annul the Circle because a Mage that she KNEW was an apostate (non circle mage) was in no way justifiable.
Whether it was justifiable or not is a whole other discussion (I think it isn't). But Anders gave Meredith that final push by killing, among others, Grand Cleric Elthina. Elthina held both Orsino and Meredith a little under control (yes, you could argue that Meredith would have 'snapped' eventually, but let's not), and her death freed them of that control.
And Meredith actually says:
"The Chantry has been destroyed by a MAGE! I hereby invoke the Right of Annulment!"
So yeah, Anders kind of let Meredith loose. And he knew it.
But hey, let's keep this discussion for another thread or time. I wouldn't want to spam Jeff's (amazing) thread with it. After all, we should be celebrating the end of an epic tale here.