Murder Sans(Dust Sans's actual name) Cannotically went insane, and starting killing after over 800 repeated Genocide runs, and since Sans is the only one who can see through resets and can travel through time and space, it puts a further reason as to why he went insane. He only wanted to overpower the human, not kill everyone. In the cannon game for Dusttale, he is upset by the fact he has to kill his own brother for the Level Of ViolencE he needs.
In the Pacifist run, if you stand your ground, not only is Murder confused by your actions, but Frisk will de-tatch from Chara, so whenever you hit Frisk, you take damage yourself. It goes on for a while before Murder stops it. The battle will end, Murder will take his hood off and say "See you in the next reset".
So in a way, you're right.
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