Was he, though? He is a liar, manipulator and user from start to finish, which means any "friendships" he had within the Inquisition are based entirely on false premises and would never have existed otherwise. If they had known who he is, what he plans to do and has already done, they would have gutted him in an instant and good riddance. For me, that deception completely invalidates any positive feelings towards him because the person who I thought was my "friend" never existed. He may have developed a degree of sincere positive feelings of his own towards some of these lesser beings, but he never allowed them any chance to do the same.
I dislike pretty much everything about Trespasser and what it means for my character and my interest in the franchise, so at this point it would take a lot to make me consider playing DA4. If I do end up buying it, though, I cannot imagine any reason not to kill him. And given the heavily pushed poor-lonely-misunderstood-Solas vibe, I resent the notion that killing him will probably be the "evil" path.
Normally I like redemption stories, and despite his many flaws I did like the character I thought Solas was ... but Bioware isn't exactly giving me anything to work with here to get me invested in the possibility of such a redemption story for him.
I doubt killing Solas will put anyone on an "evil" path, especially considering the fact the PC will most likely be someone new who has no experience with him. I think they'll offer the redemption path, but I don't believe anyone will be railroaded into it. After all, there are people who don't like him, and just to drop them into a box because they don't... I don't see it happening, any more than I see Bioware punishing people for a RP choice, since there are a hell of a lot of reasons to kill Solas that aren't evil.
The majority of Solas' lies (and I do say majority) are those of omission, and the best lies are the ones with some truth. I think we saw at least one side of Solas, and he's slipped on a couple of occasions. After WEWH for example, where he comments about having forgotten what court intrigue was like, or in the IQ's quarters with approval where he stumbles over the fact where he's seen the "subtly and wisdom". I don't think he'd make those kind of slips if he wasn't comfortable with where he was and who he was with.
And yes, if Solas' plans were known at the beginning it would have been a race to see who could gut him first: Cassandra, Leliana, the Herald, or Cullen.
But that's the whole point, I suppose. YMMV of course.