It's the respect. Solas seems to respect you and your feelings. Blackwall is shady as ****. I mean I like him, as a friend and sitting away from me. Blackwall murdered children. He didn't do it for the 'greater good'. He didn't do it because he thought it was the right thing to do. He did it, because ... his boss told him to. Also it kinda feels like he is stringing you along for one last 'hoorah' before the nasty finish.
Solas, tries to push you away and seems genuinely surprised and almost confused by his complete head-over-heels attachment to you. I know he lied and that's ... awkward, but I really don't think he wants to keep up the lie. I feel like he genuinely wants to tell you the truth.
Blackwall readily admits that he wants to actually be Blackwall. It's delusional and dangerous. He is one step away from an insane asylum. Though it is sad, he did it to himself.
He murdered babies!
It was so hard not to sentence him to death.
Re: Blackwall.
According to Blackwall if you talk to him later, he didn't know until after the fact that there were children. His problem was that he could have called a stop to the attack and he didn't. It's not that he was following orders; BLACKWALL was the one in charge there. He was trying to prop up that warmonger Gaspard. I wonder if you take him with you to the ball if Gaspard has anything to say to him?
Anyway, so Blackwall is so afraid of who he is and what he's done that he tries to become the Grey Warden who sacrificed his life to save Blackwall. And I agree, Blackwall's betrayal feels even greater than Solas. After I had played the game and seen what kind of person Blackwall really was, I personally wanted to kill him, send him to the Wardens... SOMETHING. But Uth'shiral was kind and forgiving, and at the time I needed his skills in combat. Still... knowing how Solas's romance ends, I could still romance him again repeatedly. I could never, ever bring myself to romance Blackwall now that I understand who Blackwall is.
I have similar issues with Sten and Zevran, although I typically recruit them.
But will I say that what Blackwall does is worse than what Solas has done? ... Well, considering the results of Solas's actions... no. It's just that we don't directly see the results of Solas's actions. We see what came thousands of years afterward. We don't get to see the fall of Arlathan, the thousands of free elves enslaved and the enslaved ones taken under the wing of far cruel masters. I am sure that Solas sees what Thedas is like as the direct result of his actions, however--starving elves, elves without magic, elves enslaved, elves struggling hand-to-mouth to survive. Elves who are now mortal. Spirits suffering, being twisted from their true natures. The Chantry's persecution... all of this is to him very fresh and very wounding.
And yet, I think the difference is that Solas pretty much already owns up to what he caused. And I think that given what we know of the Creators, the world would still have been in terrible shape even if Solas hadn't intervened. Maybe it would have been in even worse shape.
Solas's hands aren't clean, either. But Solas fails to tell you WHAT he is--and he has a good reason to do so--but he is pretty clear on WHO he is, via showing what he approves and disapproves of, by slipping up in his story now and then, etc. I wouldn't want to be a Lavellan who slept with him without knowing he's the Dread Wolf, since that knowledge is undoubtedly going to come out sooner or later. But, either way I think his situation is actually more workable in some ways than Blackwall's. Solas isn't pretending he's someone else; he even wears who he thinks he is in his very name. Pride.