Board Thread:General Discussion/@comment-27907368-20160514064034/@comment-26347196-20160514223753

PancryPancreas5 wrote: He knows these things buy looking at Frisk's expression. You can skip the Genocide part, because I don't think it's really convincing.

Going back to Papyrus, Sans will tell you to watch out for blue attacks when you battle Papyrus. However, if you have killed Papyrus, then reload your save point to before confronting Sans and going to the town (or just make a reset completely), he will tell you to not fight Papyrus at all.