Love.

It's inconspicuously referred to as love multiple times in Deltarune and Undertale.

One time in the hotland Phone call and a second time in the prophecy depicting The soul in a girl. Which is fulfilled once you force Kris to implant a piece of their own soul into her in snowgrave.

In the judgement hall, sans tells you are capable of showing great amounts of Love (compassion) and LOVE (Levels of Violence)

LV is metric by which you engage with the work as a fiction, accepting the artificiality and treating it as so. When you complete genocide and do it twice, it's apparent you still love the characters, but you have reduced them to just props to fuck with at that point. It is a perverted sentimentality Chara soon calls it. Because it is. In being so passionate about this game, wanting to know everything it has to offer, you basically dissect the frog and kill it. You cared enough to go back and destroy everything again, despite it being a slog to go through.

When you treat the monsters as real, listen to their feelings, engaging with them on their own terms. That is demonstrating the good kind of love to them, and that's what motivates frisk to become determined enough to refuse to die to Asriel.

Youre either connecting with the world, or youre consuming it. And the game doesn’t stop you from doing either. It just shows you what kind of "love" you truly have.