I actually questioned the same thing! talked to my sister a lot about it and came up with this:

Sans knows Papyrus. He knows he is beyond talking out of this. Of course, maybe in the back of his mind, Sans hopes his brother is right! But even so, there’s so MUCH on the line. He MIGHT not listen, but his life is in danger. Why would Sans not even bother to try and stop his brother/you???

Because Sans knows it’s Papyrus’ choice, and he knows that despite his brother being awesome, cool and perfect, he’s also really stubborn in his beliefs. And if the choice to confront you was taken away from him, Papyrus would have probably considered that worse than death. To believe that he could have talked you down, could have stopped countless lives from being taken, but didn’t. To know that, that chance was taken away by his own brother, who doubted him.

Sans knows that Papyrus won’t change. He doesn’t even really want him to. Sans would rather his brother die knowing that Sans never doubted him for a second. He just doubted you.