I understand what you're getting at, but there's a missing factor in that equation.
They are, in fact, people. They are rational beings with the ability to make decisions, and very few of them approve of blood magic and using their abilities to harm civillians.
Keep close surveillance if you wish, but not every mage is the murder machine Vivienne makes them out to be.
They are people. That's what makes the situation so very tricky. Where does public safety meet human rights?
Because while most mages don't use blood magic. Some do. And to very destructive purposes. (heck some mages use plain old vanilla magic to abusive and destructive uses)
And of course, some mages do get possessed against their will, causing enormous amounts of destruction as well.
Though I must say also Vivienne does not say that every mage is a "murder machine" She does say the rebels are foolish, short-sighted, and even selfish.
She also says that the mages rebelling when they did gave the appearance that they approved of Anders' actions, whether they actually did or not, which hurt the public image of their cause. She even admits that they have legitimate grievances against the their treatment But in her opinion, they went about addressing them in a way that legitimizes the very fears they want to quell.