Only 1-3 of my kids did anything akin to a genocide run (Jacklyn along with Clover in some runs and Frisk that one time), but if the others did of them did here are their theme/motivations
Liliana - It is my duty as a faithful servant of God to cleanse the Earth of this demonic filth. I don't care how long it take or how many obstacles I must overcome. It is God's will, and it will be done.
Brooklyn - So they plan to go to war with humanity? Tsk tsk. If only they knew just how ruthless we can be. I'll spare them the pain of finding out the hard way, I'll spare humanity the burden of having to waste supplies when this hypothetical war comes, and I'll spare whoever is next to fall of this responsibility.
Amari - If I'm gonna get past the king and go home, I need power. And killing monsters give me that power... Well, it's time to play the waiting game. I got plenty of time and monsters to kill. No use trying to get attached to creatures like that.
Lyric - These beasts are a dangerous and hypocritical race. They wish for freedom, for war, and they will kill children to achieve it. They have no morals or principles, nothing like that. How disgusting. They aren't worthy of mercy. Why show compassion for beings that show none for me? They only showed me hate and malice, they wanted to kill me like it was nothing. They murdered those poor children as if they were nothing. They murdered Amari as if he was nothing. It's as if they didn't even see them as people, just another human for them to eliminate. If they got the chance, they wouldn't hesitate to do the same to all of humanity. And yet, they acted as if that was good, as if it would even change anything... Things like them are better off exterminated.
