I can't imagine anybody being horrible enough to actually kill Bethany, that is unspeakably horrible and evil. She's my favorite character in the game, and definitely the most 'good' and noble, she's just awesome. I do plan on playing through on the templar side once just to see what happens, but I definitely wouldn't go that far, I wouldnt even watch that clip, I imagine its every bit as horrible as people say it is, I wouldn't want to see something like that. I'd definitely save Bethany if I ever played that way up to that point. Of course my 'real' playthroughs I would always side with the mages, although most of them seem to be just as crazy/evil as the templars, I'd do it for Bethany, just like I did on my first playthrough.
Another interesting point is, people are referring to the Right of Annulment, but for those who played DAO, we know that wasn't actually involved here. Even in a true state of emergency, like the one in Ferelden when abominations had literally taken over the tower, the templars, including the head templar himself, didn't have the authority to invoke that Right, he could only request it from a higher authority. The authority that Meredith had was the authority to REQUEST the right, she could've requested it from Leliana (and even Leliana might not've been high enough ranking to issue the Right, Im not sure), or from some other really high ranking person in the area, I don't know exactly who the best person in that area to request it from would've been, it probably would've been the nice old lady that Anders murdered, obviously she was no longer an option at that point, but Meredith had no more authority to kill people than any other citizen of Kirkwall. What she was doing was murder pure and simple, there was no Right of Annulment, and on that basis alone I would never side with her in any of my real playthroughs. Of course even if someone in authority had been on hand to give her the authority to execute the Right I'd have still stood against it in defense of Bethany, but it would've been a tougher spot to be in. As it was, Meredith was just a idol-crazy murderer, and maybe she would've been crazy without the idol too, who knows

. But let's not forget, what happened in Kirkwall was NOT the Right of Annulment, it was just a bunch of crazy templars trying to murder a bunch of mostly-crazy mages, it was murder plain and simple. The only mage who deserved to be executed was Anders, and he got his justice from Hawke, who was really probably the only sane authority figure left in town at that point.