Depending on how smart the mage is..yes, he could.
Mind control means you can force people to comply, without having to work around. A mundane has to gather trust and allies, he has to have a loyal army, he has to weasel himself into the royal court.
A mage on the other hand, has it far easier. A single mage can just go a location he wants destroyed, open a large tear in the veil and run away. He has an instant army at his disposal.
So yes, I do say that how easily something can be done, with how much effort and planing, and with how many people involved, does matter. A LOT.
The number of people that have to fail/be corrupt to bring about disaster is SIGNIFICANTLY smaller in case of mages.
According to your logic, Superman/Prof X is equally as dangerous as your average human, because "humanity" as a nebulous whole is capable of great feats of destruction collectively?
You're meaning to tell me that one blood mage can simply mindcontrol a vast city and do what he or she wishes? Than why haven't they? Plenty of circle and apostate mages have more than enough reasons to slit their wrists and simply mind control a city. Haven't seen that happen as of yet. It is not that easy, it is not that simple.
Sure a powerful mage can open a tear and run away. Just like the Divine can declare an unwarranted exalted march. Just like an elf can pick up some would be explosives and attempt to destroy a city block. The point is that the ends are just the same, the corpse does not care if you boiled the blood in his veins or if you drove a sword through his heart. I've yet to see a 'superman' mage or a 'professor X', maybe you mean the Somniari? Powerful mages that are extremely rare? Of which we've met One in two games, maybe two if Avernus is one.
Not to mention that the Qunari have big guns that the rest of Thedas likely is not ready for or do they not count as dangerous mundanes?
Why should they be? The Harrowing is less reliable than a drivers license.
Also, you're picking poor examples. Dalish Keepers have proven to have a rather bad influence on their clans survival. Chasind - we can only assume they work similar. The Rivani seers we know little, other than the entire Riviani circle was full of abominations.
And Morrigan is a morally bankrupt individual.
Why do it than? Why tell them that they can be lobotomized, slain, or attempt the harrowing if it is ultimately unreliable? It probably isn't a full proof test, but if it's good enough for a military mage hating organization. It's good for the rest of the Andrastians.
But have they lost control of their powers? Have they lost control to a demon? Also these are what, two Dalish clans out of several? They've proven nothing, we haven't met enough of them.
About Rivain, I did not read that, "When the other Circle rose up, the Chantry sent Seekers across the bay from Ayesleigh to investigate. They found us mixing freely with our families, training female mages in the traditions of the seers, and denounced us as apostates." - Dragon Age Wikia.
Even so let's grant that it was "full of abominations", did it fall? Did their people cry out for help? Was Rivain irrevocably damaged by the abominations or it's seers? There's no evidence that supports that. There is however evidence to support that the Chantry showed up, didn't like that they 'disobeyed their rules' and slaughtered them. Not because they were out of control, not because they were abominations ravaging the country side but because they defied and proved the Chantry wrong.
- It was quite clearly the merger with justice. He is quite clearly - and he admits it himself - becoming more and more unstable.
And my entire point was that Meredith was a very poor example, given the exceptional outside factors.
- Except she's not really one. Or should I say, was not one.
*Points finger at MAGIC*
Meredith is an example. She as Knight Commander had no business bullying the city of Kirkwall as she did, but it happened. I can credibly lay almost all of the
templar foul ups at her feet. She is incomptent, ill-tempered and ill-suited for her position. She is everything that a commanding authority should not be. And she didn't need magic to do it. But I suppose as long as it's not a mage abusing his or her power, it can be sweeped under the rug and forgotten about.
Most of the veil rending is done by mages.
I find it curious that you think locking up 1% of the population because they are extremely dangerous is horrific, but are now suggesting to lock up 99% of it.
Also, trying to lock up all mundanes is unfeasible. Logistically impossible I might add.
So what's the point of bringing up impossible propositions? You cannot lock up 99% of the population. It cannot happen. It will not happen.
Most? Maybe, definitely not all though.
Let me be perfectly clear. Locking up the one percent permanently, for crimes they may or may not commit is not or will ever be justified simply because it's possible which it shouldn't be if mages were as powerful as you believe.
Of course mundanes can't let them be.
Just like you could just let a contagious man wander freely around your house.
And thus they continually poke the fleeing tiger and then become upset when it rips them apart.