I just saw some clips of what sort of things get altered in a genocide route, and damn. Some of it is just plain disturbing enough that I wouldn't have believed it was in the game if I hadn't seen the footage myself.

If you want an example
Spoiler
Show
Here's what happens if you force Noelle to gain a bunch of levels. https://youtu.be/lz_1uVYDhBA?t=385


For a game that says straight out "Your choices don't matter," there's some major things that can go differently over the course of Chapter 2. I'm honestly a little worried to look into what a full geno run is like.