And I find it counter-productive that people here seem to confound the two topics - one of them being the first enchanter as a character (including the way he was written and all the small things that concern his character specifically) and the other the "mages vs templar - which one was a better choice as far as morality was concerned".
I've written long posts in the past on the subject of Orsino (heck, one is even quoted in the very first post of this thread) and I don't see a reason to add to the pile as no one ever took a stance towards any of my arguments. However, I would like to point out that while Orsino as a character was, arguably, a bit butchered - meaning, badly written towards the end - the clues are there and we can see that he wasn't supposed to be a 'good', morally clean, upstanding mage of the Circle. But that doesn't mean we can't like him. Evil characters (and I would rather call him amoral as opposed to immoral in the face of the little evidence we have, but even so) have their own appeal, especially when they are given enough screen time and character development for us to connect with their POV - which is not always, in the bigger picture of Thedas, a non-beneficial one; evil actions can have sometimes better consequences than the goody two shoes ones - as Lord Vetinari said once - the only thing Good People are good at is the deposing of Bad People that, they are marvelous at; however, they make really crappy rulers

. But I digress.
The point is, Orsino's actions, First Enchanter or not, are pretty minor, both in the context of the mage rebellion pre-Anders and in the context of the well-being of Kirkwall. As difficult as it may be for a player who looks at it from Hawke's POV, Quentin was a minor villain who killed but a handful of people. A very small handful compared to what Meredith would have accomplished with the Right of Annulment. And even Cullen says that "the situation here is nowhere near as bad as it was in Ferelden" and even there the Right was not invoked and many lives where saved by the hero, mages and templars both. And some of you don't have a problem with invoking the Annulment (and should I mention it was unlawful as well, since Meredith does *not* have the right to decide on that matter, as a Knight Commander? In Ferelden they had to write to Denerim to ask the Divine whether or not she would agree... since Kirkwall's Chantry is no more, here they would have to send to, probably, Starkhaven; Meredith's choice was taken in defiance, not defense, of chantry law), knowing it would include slaughtering every living being in that tower. Children included. Double standards, anyone?
So, we come to the final point of the post - mage vs templar. There is no real choice to be made in the game - we know this. And in both cases the whole thing ends exactly the same way. So, truly, we can only speculate what would realistically (because imho the ending is not realistic in any form or way in DA2...) happen were the Champion to take a stance. However that issue has little to do with Orsino, who is a single mage. This is a more complicated issue and we can be pretty sure the situation would be pretty much the same if O. did not exist and we had some random other FIrst Enchanter in his place. Meredith has always been a fan of the iron fist rule, and, as we hear in Witch Hunt, after what happened in the Ferelden tower the Kirkwall templars have gone even more twitchy. In a way, it was Uldred's rebellion which sparked the trouble at Kirkwall, and Meredith as Knight Commander, the bloody history of Kirkwall and it's weak veil, Anders, the Idol, Orsino, Starkhaven mages, Thrask, etc, etc - those were merely simple ingredients of the Ka-Booom!-wildfire pot that we got in the end. No one person - even Meredith, though she is the one who carries the most blame, obviously - is responsible for what happened in Kirkwall. It was unavoidable, and the Champion could not have possibly calmed it down - at least I can't see it happening. She/he could have, however, influenced the outcome, somewhat.
That unfortunately is quite offtopic for this thread. As is the general mage vs templar, so... Do we even want to go there?