Solas is not a hero, fallen or otherwise, and I think if you called him such, he would actually find it a bit amusing. Solas thinks of himself as the villain, and he probably has for a very long time.
And narratively, he’s not actually wrong – at least, I don’t think he will be, given Bioware’s track record with villainizing well-intentioned characters who go to extreme measures for the sake of their goals (see: Anders, who is villainzed by the actual writers and not given a chance at redemption, and Loghain, who is villainized by the narrative but not the writers, and is given a chance at redemption).
There are two very telling banters that really reveal his opinion of himself in regards, actually, so these are Solas’ canonical thoughts on himself and not things I am making up.
In one, Solas tells Solas’ his own intentions were good and that he did it to “save them” and not to “be right”, which tells you that Solas doubts his own motivations for doing things, so we can’t even solidly say Solas himself realizes he’s well-intentioned (even though he very clearly is).
In the other, Vivienne asks him if he sees himself as a villain and he responds “no more than any other clever man who wonders what he is capable of if pushed” (or something along those lines, I cannot remember his actual words at the moment and am too lazy to check). Solas does not think of himself as a hero, rather he thinks of himself as a clever man capable of a great many things others would consider wicked or ruthless if he is pushed to extreme measures, which he has been by the end of the game.
If you recall, Solas himself says he would not wish his fate upon even his worst enemy, through Cole. He does not wish anyone to walk with him because he has been pushed into becoming a villain by his circumstances.
I think it’s a very good thing to remember, that, for the purposes of narrative, Solas is currently set up to be a villain. He is a major antagonist, and even though I think he will not be the Big Bad in the end, that there are worse things than him and Flemeth/Mythal, it is important to remember the role Solas is going to play.
Is he an interesting (and to many, sympathetic) character? Without a doubt. Is he an anti-villain? Almost certainly. But right now, he’s set up to be a villain nonetheless. I would like for him to have a chance at redemption, but whether or not he will remains to be seen.