Dusttale is a Character-Centered AU about Sans. It is an AU where Sans grows a lust for LV so he can kill the human, so he kills the other monsters, eventually getting to LV 19.
Sans remembers many of the previous timelines and is extremely tired of the human always killing everyone. So he decides to kill everyone before the human can, in hopes of killing the human.
For an unknown reason, RESETs do not reset his LV, so he uses this to his advantage in order to gain more LV. At first he feels guilt for killing everyone, but over time he loses his guilt and it turns into pure satisfaction.