Not going to lie, I don't see the Knight survivng any ending except the absolute best one. Especially with what they've done and are planning to do in other routes. Whacking Tenna and Jackenstein when they didn't even have to do the latter, as well as turning Undyne into a popsicle are the type of things where the only option is "redemption through death". And just look at Asgore, he put 6 children to death and he's the central antagonist (Flowey just hogs the spotlight at the last second shut up) who dies in every single ending save for one.