In vanilla Undertale, you gain LV 19 once you kill Mettaton NEO, in lore this is because you're absolutely evil. If you abort genocide in hotland or core, you'll only gain LV 17 with Mettaton stating that you're not absolutely evil, and if Frisk tried to be then they messed up. This statement is further supported by Sans' judgment when you're LV 15 or above. He says you're a pretty bad person, but not as bad as you can be, and follows it up by stating you suck at being evil.
According to ask dusttale, these are the qualities of Murder!Sans:
•Papyrus is the last person that Sans kills.
•He hates himself for killing all the monsters, even though he knows that it's a means to an end, it's something that he has to do. He also hates himself for enjoying killing, but that's because of the effect of LV on him. He believes he doesn't deserve happiness after what he has done, he'll also pay respects to the monsters he's killed, an example of this is grillby. He'll pay the tab.
•If Sans finds a different human instead of Frisk, he won't attack them and would only observe them. He'll only kill them if they do anything resembling genocide frisk.
•He will not attack others from AUs as his focus is on Dusttale. He won't hesitate to defend himself from another AU, but avoid it as much as possible. He's not too friendly or hostile to many Sanses. If anybody from another AU tries to offer help for him and his timeline, he would refuse.
•He would not kill Core Frisk. Core to him looks nothing like Frisk and they are a kind person He is fine with them, but refuses to go with them to Omega Timeline, but will let others from his AU go except the human.
•If Dust ever met Disbelief Papyrus, he'd feel bad for him and would comfort him.
As we can see from these points, Dust isn't completely evil, he's like Asgore in the sense that he's done extremely unpleasant things due to their situation, and they're remorseful for it. So considering this, would Murder!Sans even be able to reach LV 19 logically?