You forget, that DA is a Medieval setting at best. The concepts of human rights were vastly different back then, and you have to take it into account, when you gauge some conflicts, that has already happened. On the other hand, the concept of retribution were very familiar, as well as religious fanaticism, and if combined... So, i daresay that Meredith was right - it doesn't matter if you spared or executed Anders, the Kirkwallers would have torn Circle to shreds as well, as any apostate who wouldn't fled the city. Would be the same as Annulment, just with much greater civilian costs, aka lives. Kirkwall doesn't have BBC, CNN and the likes to calm down the mob.
Stop judging from the height of 21th century, if you want to understand Thedas and its mentality. I don't say you have to support it or accept it, but ignorance is shallow.
Considering the Gallows is surrounded by water and needs to be reached by boat, I don't see how every Kirkwaller is going to attack the Circle. I also think it's absurd to justify the mass murder of an entire population of people on the basis of appeasing a hypothetical mob, even by the standards of Thedas.