How many people allowed Solas to kill the mages? Or even continued the relationship? That should have been the point to turn away from him if ruthlessness was to be an issue. He was willing to kill. For no reason other than anger. That's a trait of character. If you kill 5 or 5000 people does not really make too much of a difference anymore, I think. Especially since the mages-killing was very close up (seeing their faces, their fear, their helplessness, and knowing of their ignorance), while the conclave was neither planned nor anything personal. "3000 people died in a plane crash" surely is bad if you were the one not checking the engine correctly, but not as emotional as killing one person you can look into the eye while doing it.
But it wasn't just anger, not... simply anger, at least. It was injustice, in that case.
Preserving the balance of Order and Chaos that makes free will possible requires death. Sometimes the only people who have to die are unarguably Bad Guys. Sometimes, though, innocents have to die so others have the chance to live. It's the trolley problem- the necessary consequences of absolute pragmatism: the end justifying a means that is in itself always morally Good, morally Right.
There is a runaway trolley barreling down the railway tracks. Ahead, on the tracks, there are five people tied up and unable to move. The trolley is headed straight for them. You are standing some distance off in the train yard, next to a lever. If you pull this lever, the trolley will switch to a different set of tracks. However, you notice that there is one person on the side track. You have two options:
(1) Do nothing, and the trolley kills the five people on the main track.
(2) Pull the lever, diverting the trolley onto the side track where it will kill one person.
Which is the correct choice?
This is an imperfect mapping, of course, since in DA Universe/Solas/Blight terms, making the "wrong" choice would mean sacrificing the ability of the Individual to ever choose again. It highlights the basic idea, though: the idea of "necessary" deaths, and their weight on his character.