Oh for crying out loud, you have said this a million times in different threads. Letting you murderknife like in DA:O or do even more selfish-power hungry actions wouldn't violate their moral code, at all.
Simple. They tolerate the warden's murdering people around for his benefit because their moral code values "everyone not getting eaten by archdemon" higher than "a few people got murdered for selfish reasons". That's it. Hard truth, your companions are adults, they swallow it and move on. Killing you for your actions would result in worse things than the ones you do. Leaving you and not helping you with the archedmon would have the same problem. Only doing something monumentally extreme and personal to the companions, not just murdering people they never met, should result in them turning on you and not just disapproving. They don't live in a disney story, they should be able to view past their disapproval to the end goal they themselves value more.
And just like that happened with the warden who was ferelden's only hope against archedemon, since non-fereldern wardens weren't around, it could have happened with the inquisitor who was the only one who could seal rifts.
We can discuss why bioware chose not to have evil pc choices and if we like that or not, but saying that allowing for evil choices nullifies companions' personalities, unless they turn on you is just plain wrong.
There's absolutely no reason - beside your protagonist shield - why they'd keep you alive in preference to Alistair. The game works in part because Alistair is spineless.
And now when we reference the murder knife we're talking about petty, psychopathic evil. Robbing wounded elves? Killing helpless prisoners? That's not done for the sake of stopping the blight. That's done because you get off on causing pain.
There's absolutely nothing the Warden offers that justifies not killing him. It's all plot contrivance.
Alistair is as LG as you can get. The same with Leliana and Wynne. Yet they stand by as you get your torture fetish on because ... reasons? It's nonsense.
You have no value. Alistair has the treaties. No one knows GWs have to strike the killing blow against the AD. Hell, even with 0 Wardens the party had the treaties and could just Blackwall the whole thing.
When Morrigan suggests participating in a blood magic ritual proposed by a Tevinter magister Wynne should be killing you alongside Alistair. Except - like with her outing you as a BM - it would stop you from completing the game. So the game ignores it.