The human is ready to fight sans then suddenly a time paradox  happens and then another sans shows up he is called pacifist sans because he’s from the pacifist route a timeline which frisk spared all the monsters and brought peace Pacifist sans is confused sad and blinded by hatred he seeks only revenge for the false happy ending he was given genocide sans is called genocide sans because he’s from the current timeline in which frisk kills all the monsters and he’s the judge ready to stop them