The 'Genocidal' Storyline of MentalTale doesn't involve as much death as you may think. It begins the same as a 'Pacifist' Storyline would, with Frisk falling into Ebott and being taken into UMH. Out of pure nervousness she would treat Toriel and Asgore with kindness, as they are bigger in stature (and therefore more intimidating). However, once she would come across someone like Papyrus, she would treat them awkwardly, and unintentionally ask harmful questions. After a short while of getting her bearings in UMH, Frisk would start to become more comfortable with being rude to the UMH residents. Eventually she would meet with Undyne, and treat her in such a way that she would not only try to kill Frisk, but possibly herself as well. When Alphys hears word of this, she becomes enraged and starts plotting ways to kill Frisk. Sans may also get in on Alphys' plans, if Papyrus had been hurt.
This run generally ends in Frisk's death, though she can die in several ways.
~More info to come~