(heavy breathing) where to start..... where to start...... this opinion is coming fresh from a recent genocide playthrough, but the 1# thing I wish people kept about Papyrus is that he's undoubtedly kind and merciful!! Despite being face to face with his impending doom he chose mercy! And even in his dying breath he still chose to believe in the potential for you to redeem yourself! Sure he's extremely naive, and doesn't have a lot of life experience to boot, but I think even if he were to go through unimaginable horrors, he would be the type to pick mercy regardless.
So as cool as some of the fan interpretations of an edgier Papyrus battle can be, he would do it simply because fighting is fun! Not because he wants to genuinely harm the human!