But a more comparable example, I think, is Shepard at the end of Mass Effect 3, especially if Synthesis is an option. No matter what you do, you're changing the galaxy, probably murdering countless people, and maybe even altering the basics of life itself. Does that mean that rejection is the only moral action for Shepard to take? Does that mean they should just sit themselves down in the middle of the Crucible and wait to be processed, because they don't have the right to play god? Should they just give up, simply because there's no good option?
Well, Solas is the Shepard in this equation - and his only real options are Destroy or Reject.
So when a human character does it it's fine, but when a non-human character does it's heinous? That's... fairly typical of BioWare, and fiction in general. 
I just realized, from what I can tell here most solasmancers seem to create at least two (often far more) characters to romance Solas, and I totally get that because then you can explore new aspects of their relationship... but I've never been able to romance my favorite characters with new characters! For example, despite having 7-8 different world states, Alistair is only ever together with my canon Warden, and the same can be said for Fenris and my canon Hawke. Solas is no exception to this rule. It's not a jealousy thing or anything like that, I love to encourage people to try these romances! The more people to discuss and gush with the better! Am I the only one that has this weird quirk?
Nah, I'm the same way.
I romanced Alistair with my "canon" Warden, then never really felt compelled to romance him with another character because I loved that one so much. I also romanced Solas with my first Lavellan, and haven't even wanted to romance him with another because I just like that one couple, relationship, dynamic, etc. so dang much.
I think I get bored quite easily playing magnanimous idealists xD
Ah, I just get disappointed. BioWare loves playing the "idealism is for idiots who will be punished accordingly--except the human noble PC, whom we'll reward for their idealism and ambition" card, and because I never learn I just keep playing the idealist after idealist card and then curse BioWare for losing.
I imagine my City Elf Warden eventually became suicidally depressed years after the Blight (part of why she dropped everything to search for Morrigan in Witch Hunt and went to find a cure for the Calling in DAI, because part of her hopes to disappear or die in the search), partly because she had truly believed that being made a Grey Warden and becoming a hero would improve things for her people, but the narrative and end slides just kept beating her (and me) over the head with how life isn't fair and one elf becoming a hero doesn't mean elves everywhere will be treated better and you've just got to live with it.
Surprisingly though, making Lavellan a magnanimous idealist worked out way better. She's made of way tougher stuff than I or my City Elf, so no matter how much sh!t this game keeps throwing at her for being an Elf/Dalish, she never stays down for long.
Solas: "I begged you not to drink from the Well! How could you not listen? You have given yourself over to the service of an ancient elven god!"
Lavellan: *shrugs and points to Mythal vallaslin* "Could be worse, I could be the servant of a human god--oh wayt..."
Solas: "Your vallaslin, they're slave markings."
Lavellan: "Used to be. The Dalish have reclaimed them and they mean something else now.
"
Solas: "I can remove them."
Lavellan: "No thanks, I'm good.
"
Flemythal: "I have no orders for you... yet. *leaves ominously*
Morrigan: (thinking) "Thank Whoever I let you drink from the Well."
Lavellan: (thinking) "Thank Mythal I didn't let Morrigan drink from the Well. Now I'd better get moving using the knowledge of the Well to help my people while there's still time. Gotta get mileage out of this price of eternal slavery before she comes back to cash it in..."
Leliana: "I'm sorry Inquisitor, but Solas is gone."
Lavellan: "That's okay, I'll find him.
"
Solas: "The elvhen gods were not gods at all."
Lavellan: "Well, if that don't beat all."
Solas: "The Dalish were half-right about the legends."
Lavellan: "SEE?! We were half-right! And you acted like the Dalish didn't know anything..."
Solas: "I must destroy the current world to restore the old one."
Lavellan: "Let's not be melodramatic. There has to be a less destructive way you haven't thought of yet."
Solas: "Vhenan, I am an ancient, immortal elf who has spent eons in the Fade, brainstorming a way to do this delicately for centuries. If there was a less destructive way, don't you think I would have thought of one already?"
Lavellan: "You're just being grim and fatalistic! There has to be a way you haven't thought of yet. If we sit this one down and put our heads together, I'm sure we'll come up with something."
Solas: "Vhenan..."
Lavellan: "I recommend starting with the sitting lotus position, followed by some meditation to clear our heads, then we'll munch on some nuts to increase brainflow..."
Solas: *lifts the magic holding the Anchor back to end the conversation*
Then there's Lavellan's closing line: "We'll save our friend from himself."
It's almost funny to play a character who is just too dang optimistic (and in some cases self-deluded) to indulge in any of the angst BioWare keeps trying to throw at her. 