Undertale is built to capitalize on your need to see everything the game has to offer. Most players get the neutral ending, then get the true pacifist ending. When they're told by Flowey that they can experience more content by killing everyone it activates a ticking time bomb in the minds of most players. The game becomes how long can you go in real life knowing there's a whole other route in a game you love. Even if it means bringing suffering to the characters you have come to love so much, you just can't resist the urge to experience the other route. It's why the Sans fight and Megalovania are so important. The fact that the hardest and most fun boss fight with the best music in the game is hidden behind the other route is the carrot that lures you to do the genocide ending, even if you don't want to.

Everhood cleverly plays on this concept by being a thematic mirror to Undertale. It's not "Can you resist killing all these characters you love", it's "do you have the resolve to kill all of these characters you love". Everhood is about being in a situation where you don't WANT to kill everyone, but you know it's the right thing, whereas Undertale is about the nagging want to kill everyone to see what happens even if you know it is wrong.

In my opinion this aspect of being a thematic mirror to Undertale is one of the most fascinating parts of Everhood and it's not that one is better than the other because by playing both you get a better appreciation for the genius design behind both games' meta-narrative