The thing is you didn't go through the game with the option of "redeem" or "kill", Loghain.
I get that you will try and show him the plan won't work or is unnecessary to achieve his aims but it would have been helpful if he had answered the question of why it was necessary. (Clearly that has more to do with the plot of subsequent games).
What I have always objected to is the reason for taking this line and the words put into my mouth at the end "We will save our friend from himself".
The emphasis and intent are different from the how you are led into the choice, to what you actually say.
All those fine sentiments he expressed in the same game and which I identified with, mean nothing now that he intends to go through with his plan even though he admits he sees us in a different light now and that in his eyes we "deserve" better.
For example, you prove to him that his plan won't work; so he abandons it in favour of another. He is not "redeemed" since he was still entirely willing to go through with the first plan but common sense has prevailed and the world is saved. In his eyes his plan was never "wrong" in the sense of being "evil", simply wrong in the sense that it would have failed. So really what I am trying to do is "redeem" the world from Solas.
True that your main objective most of the game was to defeat Loghain, and also true that you had more options than just kill or redeem. But I'm sure Solas' story would go the same way, except you've made your main choice already (which I'm sure we'd be able to change at any point).
I'm not displeased that he didn't tell the Inquisitor the answer. Obviously BioWare doesn't want to reveal too much to players, and it's interesting that Solas will leave it to an already enlightened Inquisitor to figure out. I prefer this actually, and already have some theories. Maybe Solas thinks the Inquisitor's search for answers will in itself convince him that Solas' plan is correct.
But the full quote is, "We will save our friend from himself, if we can." So killing him is still on the table, it's just that you will try to reason with him first.
The emphasis/intent and what is actually said are different, and this is annoying, but this has been the case with almost every dialogue choice in the game.
You're right that Solas is a hypocrite who has gone back on the virtues he espoused. Maybe he knows he's immoral, but holds the restoration of his people as more important. Honestly I don't know if this is good writing or bad writing on BioWare's part. But his meeting with Flemeth shows him saying he should be punished, except that he still needs to help the People.
"Redeem" might be a poor choice of word, but it depends how the story plays out. Maybe the (ex)Inquisitor convinces Solas that what he's doing is immoral and against everything he stood for, and it isn't worth rebuilding the elvhen empire on the corpses of millions of beings he once recognized as people. Because how is that any different from Tevinter sacrificing slaves to empower magic that creates their buildings? BioWare could still add some interesting twists to this plot.