Personally, I get super upset when I hear about poachers killing dolphins, bears, wolves, etc. but that does not make me any of those things
I think it is possible Solas is upset about the killing of Old Gods because they are his relatives, making them either Forgotten Ones or part of the Pantheon, but it is equally likely Solas is upset because they are part of the past that Solas values over the present. One of your first conversations with Solas he can say something along the lines of, "I would have suggested a griffon, sadly they are all extinct," when suggesting a mount for the Inquisitor. Of all the creatures, Solas chooses a griffon, something that was more common in the past and was made extinct by the actions of the Wardens.
The game is definitely trying really hard to make the Old Gods and Elven Pantheon connection (the constellations), and Mythal taking the form of a Dragon. While I think this might be foreshadowing, it seems pretty heavy handed (and all the reveals thus far have been subtly established), so I am inclined to believe there is an important piece missing, as you'd think an OGS Kieran would have given some hint about Solas, or you'd think Cole would imply some of Solas' friends trapped being dead or absent. Because of all the OGs who have risen to cause a Blight, only one soul is possibly still alive, and maybe a second if you buy into the whole Andraste is Dumat thing. This leaves only two of Solas' friends he trapped being around, you'd think Cole would draw attention to that instead of talking as if they are all still there. But there could be other pieces missing.
There is another issue that no one really goes into. We don't know when the Old Gods were sealed away. Some assume it must have been pre-fall of the Elves or coincided with the sealing of the Pantheon, which we don't even know exactly when that occurred in relation to the rise of the Imperium. The Elves had an Empire that definitely existed in some capacity for thousands of years and we only know that the Pantheon disappeared some time before Arlathan was buried under a Blood Magic ritual by Tevinter. We have no idea when the Old Gods were sealed away, but if you believe Tevinter legend, the Old Gods were active in the early centuries of the Imperium. Or, if you believe that the betrayal Mythal talks about had something to do with the Old Gods, considering her apparent sympathy with wanting to keep their souls alive (the dark ritual) and her connection to dragons, then this happened before the sealing away of the Pantheon, as Mythal was dead and could not have been sealed away by Solas prior to his trapping the other gods.
Another thing to consider: whose side Mythal is really on. She seems okay with Solas at the end, but she's got a whole vendetta going, and she appeared to be trying to do something before Solas arrived. If she trusted Solas, or aligned with his purpose, you'd think they would have worked closely together when he awoke. Mythal as Flemeth has also had numerous dealings with the Dalish for centuries. The Dalish also have a surprisingly accurate description of the spheres their Pantheon ruled over, considering how little else they know, except for one God, Fen'Harel, who went from being a god of rebellion (Temple of Mythal) to be described as a god of betrayal and deceit. This could be their bitterness at Fen'harel for eliminating their gods, who were also potentially their protection and source of stability, but perhaps Mythal, the only member of the Pantheon thus known to have existed in Thedas and had contact with the elves after the fall of their Empire, who seems obsessed with a betrayal, had a hand in shaping how the Dread Wolf would be remembered? Crazy theory I know, but I'm suspicious why everyone assumes they must be on the same side.
Another thing about Solas to remember: he gave the Orb to a tainted entity. If you believe the whole, "Fen'Harel sealed the Pantheon away to stop the spread of the Blight" business, why did he trust an entity who was marked by the Blight? Perhaps Solas' relationship with the Blight is the key to understanding his disdain for the Wardens, and why he seems to offer very little insight into the Blight we did not already know despite dropping lore bombs all over the place.