Here's my take on this question of killing Solas.
In my headcannon playthroughs of each DA games, my PC decisions are based on what would make Thedas a better place in the long run. So in the next DA game with Solas being the main villain, my decision will again be based on this.
Currently, the Thedas we have I find to be a shitty one. Elves as second class citizens, magic being feared and hated, so very little progress in society in general. Ancient elves has done so much with magic that none since them has been able to replicate, not even Tevinter. Imagine what the eluvians can do to commerce?
I want this to change. So what I'm saying is if killing 90% of the population means I would get a Thedas that would treat elves as an equal race and magic as something to be respected and actually used to improve the lives of Thedosians, then I would most likely join Solas. If that option is even available. I hope.
In this fictional world in a video game, I am God. Not Solas. We all have are own ways of role-playing and that's fine. One of the things I love about DA is the multiple endings and we can pick and choose what endings we want.