Who actually sides with Meredith?

Yes, Meredith was utterly out of her god damn mind, completely failed at her job, and the problems with the Circle were mostly her fault.

But, practically every mage we see is a blood mage/abomination. We find out the blood mage responsible for killing Hawke's mother has an accomplice in the Circle. By act 3, blood mages and abominations (Grace, Huon, Evelina, the Resolutionists, trash mobs mind controlling gangs to do their bidding) are pouring out of the Circle and Orsino suspiciously attempts to cover any and all evidence of this. He suspiciously throws a fit when Meredith tries to do something as reasonable as room searches in light of the overwhelming evidence that there are blood mages in the Circle.

MAYBE I wouldn't have sided with Meredith if the Circle hadn't made it abundantly clear that it was rotten from top to bottom.

My second reason for siding against the mages is that there's also the perceived futility of trying to defend them. Excluding metagame knowledge of knowing you're 100% going to survive no matter what because the Templars stand down, you know reinforcements will come. Before that, we thought the Divine was going to call an exalted march on Kirkwall. 100 health potion gameplay mechanics aside, Hawke and co. can't survive an entire army so the safer option would be to comply.

I sided with her once or twice (out of like so many playthroughs) but it kind of makes sense if you play a mage in a certain light. First of all, there truly are dozens and dozens of blood mages in Kirkwall - obviously caused by Meredith. Yet, although Meredith caused the conditions that gave rise to blood mages, it still holds that the right of annulment exists to purge a circle infected by blood mages. Within that world, the right of annulment is an inevitable although tragic and avoidable consequence of the blood magic infecting the circle, which is, of course, a consequence of Meredith's actions. It is a rule.

"Anders forced our hand."

Per Cullen, "even just one [Maleficar] could influence the mind of a King."

Although we slay blood mages like dime-a-dozen mooks, if you uphold the lore established in Origins that a blood magic infection in a circle is really, really bad and terrifying and cannot be let loose no matter what, then even mages agree to the right of annulment as a rule.

It's a doomsday machine, one which cannot be stopped even though the instigation of its gears may have been malevolent.

But that's just it: the circle mages are just about the only mages in Kirkwall that AREN'T blood mages. It seems to me that most of the blood mages you encounter either escaped the circle and then turned to blood magic in desperation or for revenge, or were never in the circle to begin with.

Plus, ultimately it's not the blood mages that make Meridith invoke the right, it's Anders' actions. And he isn't even a blood mage!

Oh that's true.

Damnit.

Don't forget that if Karras is still alive, he reveals that Meredith already tried to invoke the right of annulment at the start of act 3 which means Meredith was just looking for an excuse to kill the mages,

I like her, really good character.
But you play always a hypocrite mage Hawke? Or what's your Hawke logical reason? indestructible love for Meredith?

But she doesn't just plan to keep them in line, she plans to murder every last one of them. Most of them haven't done anything wrong.

After the dirtbag, that calls himself Anders, murdered everyone in the chantry.