Board Thread:General Discussion/@comment-10452902-20170507091901/@comment-31196611-20170507102735

Sans knew the human dying would allow things to be reset quickly, so theres already that reason, and besides its a fitting karma, "The Human" knew sparing Sans and getting killed is basically the same thing that happened to Papyrus, he offered them mercy and they killed him for it. Its the Humans fault for not seeing the error of their ways, and still trying to kill Sans after.