It's not like Frisk is the first human to go tromping through there, and it's strongly hinted that some those humans weren't weren't terribly diplomatic (the old tutu 'covered in dust' or the empty gun, to use some examples). Also the 'he's just a kid!' argument falls rather flat in a world where human children can become immortal spirits of murder.

This whole scene kinda tainted my opinion of Sans. I mean, I could understand if he did this post-genocide, heck, I could even understand him doing this if you ever killed a monster, because all monsters have families and lives and hopes and dreams and stuff.

But in this scenario, he's kinda just scaring a little kid who is lost and trying to find their way home. Given that for this to occur, they probably had to be nice to his brother (i.e. not totally ignore the puzzles) doesn't really help things much.

Not saying I don't think he's great in a lot of other ways, and I think this was a good idea, since it gives the character more depth, plus it hints at how much of a bad time you could have on the genocide run.

Buuuuuut… that doesn't mean I don't think it was kind of a jerk move, though.