Gaster infects Sans with one of his DT experiments. Instead of altering his body, the determination alters his mind, removing crucial parts of it, namely Happiness, Confusion, Common Sense, and Empathy. Upon meeting Frisk (Was originally Chara but was later retconed), Sans discovers the power of RESET, the ability to commit murder and get away with it. upon the realisation of this power, he somehow gains the ability to Reset and kills everyone in his timeline over and over, enjoying every moment