@singing-robot not to mention they are polar opposites so they are going to have a good bit of disagreements. Believe it or not real siblings argue and aren't perfect but that doesnt make them terrible. Sure they both could improve but they still love each other none the less! Even with Papyrus trying to get Sans to exercise and Sans helping Papyrus build his battle body, in a way I kind of envy there relationship :3
I don’t think that Sans is entirely a bad brother. Or even a bad person. A bad friend? Maybe. The same goes for Papyrus, even, for the people who argue it. But I think many people compare Papyrus and Sans’ relationship to a relationship between close friends, or they’re building up this huge idealistic thing that hardly any siblings will ever have, and certainly not those two.
Sans speaks a little down about his brother and refers to some of his habits/quirks in a childish manner? That’s just what siblings do, fam. He’s not gonna see his brother as this sweet, adorable figure, because he has to live with him.
Papyrus tells lies around Sans to seem a little dumber? Maybe he’s going to have the upper hand and completely blow Sans’ mind one day. Besides, half of it is for kicks and giggles, a joke system only between brothers. A constant set-up for jokes and lame comments.
Sans promised to “watch over” the human and supposedly lets them kill Papyrus? Listen, man, Papyrus is an adult, and a pretty strong one at that. They both may have underestimated the human, and Sans isn’t the loyal guardian who’s pledged a life-debt to his brother.
Papyrus seems less happy around his brother? Do you have any idea how much of a drag it can be to constantly be near one person who never leaves you alone and is a total pun-ttention hog??? Honestly, he needs a break.
TL;DR: Neither one of them is a bad brother, they’re just siblings who hardly get a break from the other and it shows.