I am not so sure. Yes, it is grief of course, but that's the shallow answer. Not everybody goes on a killing spree when grieving. I felt he could not understand their mistake was ignorance, fear, weakness. He would not act like that (probably) and he sees himself as the moral authority, the one being able to decide what is good or evil. I felt that was why he acted like that, because he decided they had been evil and decided they deserved death. That's pride for me, not being able to step back and look at a situation through the eyes of somebody else.
This was a bit back but I have to say I disagree with this. Yes, Solas is prideful but I don't think everything he does is because of pride. These mages just killed a very old and very good friend. He was seeing red, the way most people would be seeing red. I'm not sure pride had much to do with it. He was getting his rage on, and to his credit all you had to do was say his name to get him to back off. I think he realized that he was going to kill these mages in a moment of passionate anger and that there was no other purpose to it than pointless vengeance, so you remind him of it and he stops.
If that's pride, then I'd say every single person in that situation would be prideful. I sure as heck wouldn't be thinking about someone's motivation if I saw them standing over the dead body of my very good friend no matter how many rapid-fire excuses they make. Knowing they tortured and killed someone I cared about cuz ignorance wouldn't make me feel better in the least or be less angry at that person (Circle mages being ignorant of the dangers of summoning and binding spirits makes me side-eye anyhow. I think they knew they shouldn't be doing that). It doesn't have to do with being a moral authority in my eyes. Sure, maybe Solas thought the way you think he did but I personally feel that's reading too much into it.
Edit: I don't mean that everyone would kill those people, but Solas is in a game where he and all his companions kill people all the time so it really isn't that big of a deal in that setting.