Regardless, attacking the Circle for his actions is wholly unjust, and then claiming that the Circle supports mass murder because of it is just bizarre. I have no intention of siding with Vivienne's "loyalists" if it comes down to it; sadly, she doesn't give us the opportunity to see this gigantic logical fallacy before recruiting her, so I can't use it as RP justification for not recruiting her.
I didn't see her as claiming the Circle supported mass murder; just that they appeared to when they took up his cause. As a powerful woman in Orlais and, frankly, as a human being with any common sense, Vivienne understands that appearances matter in terms of how people react to you. And that people's reactions matter in terms of goodwill. I'm not saying perception is reality, but I am saying perception matters in reality. That's just how the world works because you can't generally read people's minds and intentions or even see that they are fixed.
We have never been shown that however, in the games mages outside the tower are hunted and killed, unless they are operating under templar supervision to earn the Circle and the Templars money, or have the 'companion immunity' all others are fugitives. I have to go by the games, and they show brutal execution after extreme torture for leaving, if taken alive.
Bleachrude gave examples from the games. DA2 obviously has no examples (though theoretically if you are Mage Hawke, Meredith totally allows you to live outside the Circle and knows about you; it's implied they even know about Anders and allow him lenience because of Hawke and his Gray Warden status) because it was all about an extreme Circle. The game tells us the particular Circle is extreme in several places. The most notable is the mage who helps the children but becomes an abomination (no idea her name) and tells them not to follow; she went to the Circle for help, thinking they would help her feed the kids (non-mage, it seemed) and get on her feet, but not lock her up, based on her experiences with a Circle elsewhere. She was truly surprised when Kirkwall Templars were like, "Get in the Circle, apostate" because that's not how they roll where she's from.
Even DA2 doesn't show that most mages who run away and are returned to the Circle are executed or made tranquil. Only those who turn to blood magic are. There are several mages you can return who do not get made tranquil (you see them snipe at you in the courtyard later) or killed. And that's in Kirkwall! Awakening also shows us that Ferelden's Circle has much tolerance for Anders who basically has perpetually escaped and claims he will just try again if they take him back, yet he is never made tranquil.
Edit - The exception to this is Karl. But I don't really know the whole deal with Karl. He does seem to be made tranquil for a very minor crime, but then there are others who I return who aren't made tranquil, tortured, or killed.