I usually stay away from topics like this , so I don't say something that offend or hurt someone. But I have to say this. Him killing these mages is straight up murder ,IMO. He does like to dispense justice as he sees fit, but these mages...he looked straight at them and killed them. I doesn't feel right. They didn't summon the spirit to torture information out of her, or to do some sinister ritual. They needed protection. They were ignorant , true, but they didn't deserve to be murdered.
Killing Howe is another story . His intent was evil, born of greed and jealousy, he was a horrible person,a traitor and a killer. Vengeance upon him was justified.
I have yet to actually let Solas kill the mages in any of my playthroughs, but while it is murder (all vengeance is), I can understand where he's coming from. One of his oldest friends just died as a direct result of their stupidity; I can completely understand wanting vengeance for that. I usually have him stop, though, because he's generally a very level-headed, thoughtful individual, and I don't feel comfortable encouraging him to rain vengeance down on everyone who makes him angry even if in this instance, I actually feel that it could be justified under certain viewpoints (Solas certainly sees it as justified).
Now, if I were playing my canon Warden....she was very much on the vengeance train (from what I remember), but she was a city elf so had plenty of reasons IMO. I tend to play my female Lavellans more try all options that don't result in execution/murder first...except for totally murder knifing that one guy from Adamant because I was mad about having to pick between Hawke and Alistair in the Fade...so I took a little vengeance in my first game. In hindsight, I should've probably been more open to Solas frying the idiot mages considering my own vengeance execution later on, but meh. LOL. Also, those were Kirkwall mages, so maybe their stupidity is not really curable? I mean, I still facepalm about what Orsino did even after I sided with the mages. Kirkwall mages, what is wrong with you?! *shakes fist*