But the characters in the story aren't debating whether her decision was or was not possible to justify under a utilitarian framework. They're reacting to the (new-to-them) knowledge that she's evil, and they get that by observing the carelessness with which she coerces a child into experiencing torture.

You might think that they should have already known that Addy's evil, but of course Jake has previously been under magical influence leading him to like and make excuses for the Volturi (including Addy), and Maggie is probably not inclined to write a vampire off as evil "just" because they murder people, since she spent a great deal of time doing exactly that herself.