Presumably if the basis for calling for an Annulment is dubious, Hawke is not on such great ground either. While it appears that the decision to Annul a Circle is discretionary, presumably that discretion is fettered. It's not entirely clear Meredith would have had the authority to declare an Annulment anyway, despite being authorized in principle to order it following the death of the Grand Cleric.
So participating in a massacre that could later be declared illegal would not necessarily assist Hawke either.
Except it does. We know this from canon, because Hawke's who side with the Annullment get to become Viscounts. Hawke's who oppose it don't. The Annullment doesn't become more legal just because Meredith dies in the course of it.
It's not even a matter of 'helping' Hawke, as if it provides a boon to them directly- Hawke just needs to be the last (wo)man standing once Meredith is weakened. In a zero-sum arrangement, Meredith's loss (Chantry response) is Hawke's gain so long as Meredith loses more.
Hawke's power base doesn't derive from the Chantry in the first place though, nor does Hawke require the Chantry's sanction or even forgiveness. Hawke is Champion of Kirkwall, and has been a 'stay out of jail free' member of the nobility even longer. The Chantry doesn't really have a leg to stand on if it tries to go after Hawke as well as Meredith, nor is there much reason it would want to- no more than it did when it didn't even stop Hawke from becoming Viscount, and not when Hawke is going to be politically key for removing Meredith from power.
Hawke did not expect gratitude from Meredith, nor gratitude/support from the Chantry. If Hawke is a mage, because this fact, if Hawke is non-mage, then Bethany may will die. Annulment, remember? And Meredith is lunatic.
...and this has absolutely nothing to do with what I said, since what I said in no way required or even suggested a need for gratitude or support.
Meredith is going to be in hot water after the Annullment one way or the other. That's how the Annulment weakens Meredith. That's the answer to SD1's point.