You directly ask him why he saved them if they will die anyway. His first reply is to tell you just because they have to die doesn't mean he wants them to suffer first. Aka he saved them so they wouldn't suffer. He prefixes his "they offended me" with "also". The term "also" means something came before. Its an extra point added onto the "valid" reasons he had to kill them. You are taking the line out of context. Not to mention, you only get that line if Solas actively hates you and you didn't convince him people were real.
Solas does not kill people because he doesn't like them. He'll certainly kill people for some really bad reasons (aka "let's murder millions of innocent people to tear down the Veil"), but not because he dislikes them. Murdering people for your own benefit is the type of thing that loses you influence with him.
To me it sounded like he was saying their inevitable death is no reason not to save them. Like he was arguing your logic. But whatever the case, even if preventing suffering was a reason, it wasn't the only reason. The Qunari offending him was a reason as well, and this is relevant to the ex-Inquisitor who also offends Solas.
I know this is only for a Solas who doesn't see people as people. That's my point. Firstly, this goes against your theory that he didn't want innocents to suffer. If they aren't people to him, then he doesn't care. Second, my only point here is that a Solas who negatively approves of the Inquisitor only spares him because his death would cause more unnecessary chaos. So if that were no longer the case, then Solas would have no reason to not kill the ex-Inquisitor, and probably would based on the prior offense.