I've done some more thinking and thought of some more fun parallels to Homestuck in Deltarune. Mainly, involving this.
In Homestuck, two governing forces of the universe arbitrate whether a God-tier character (Final form) has what's called 'conditional immortality'. If you have achieved this status, you may only die if:
1. Your death was heroic (in service of others/for the greater good)
2. Your death was just (fair in terms of your odds of success/a risk taken, a life lost)
The obvious joke people might make when looking at this might be to say 'OmG sPaMtOm CoLoRs!!11' but part of me thinks it's deliberate unironically. If you've seen my previous posts, you'll know I believe that Deltarune represents a systematic simulation Gaster runs to achieve an ideal outcome, that being saving the world. And if you've seen me say that, you'll know that I believe:
1. The Devil (Smile/Friend) represents the forces of the universe preventing the desired outcome, that being the heroes fulfilling The Prophecy.
2. The Prophecy is a list of instructions in which we the player (The Angel) must help facilitate in order to save the world.
I never previously gave thought to why The Devil and The Knight operate in the way they do without punishment, but I think I've got it. The heroes fight an uphill battle with two resounding messages at heart -
Nobody chooses who they are in this world - Kris is forcibly made the vessel without our choice (aside from us playing the game), tragedy struck hometown, Dess disappeared (and in my theory became The Knight with a similar possessive influence as us, the player)... In the same vein, the forces of evil in this world have no choice other than to do what they do. The Devil/The Knight desires The Roaring, Gaster desires the experiment to succeed... Both are seemingly undefeatable forces who act antagonistic - not heroic, maybe even anti-heroic, or *villainous*. In The Weird Route, we feel immortal because we make evil actions to grow powerful, we become 'conditionally immortal'.
Then, there's the notion of your choices not mattering. The world decides externally of you what is right and what is wrong, and they are *powerful* because they are *free*. We are powerful similarly because we make our choices matter, we make our power known by kindness or force - and we break the world as we see fit to get the outcome we desire. Because we the player can make meaningful choices - because we can save, and have determination - we are 'conditionally immortal'. This is *unjust*. It's not fair that those in the world can die while we break the rules and persist indefinitely. And that brings me to my final few points...
Conditional Immortality in Deltarune is the inverse of Homestuck's version of it. And... I think I know how we can represent this.
The colors yellow and pink represent 'injustice'/'unfairness', because in Undertale you choose whether to have pink or yellow spare text. They do the same thing, though, so the choice doesn't matter. Choices not mattering ties back into 'unjust' fates as we talk about them. The Devil makes you believe that you can achieve freedom, but making a deal with them sacrifices who you are - like in Spamton's case. The Devil is already correlated with Gaster and 666, so them being antagonistic forces together feels fair, but this point drives it home I think. The bottom line is that, when you rob fairness from reality, you can profit off of it and live forever - like The Devil.
Second, I believe 'villainy' is represented with holes. If we parallel Endogeny's face with Smile/Friend, we can put together something equally fascinating. Gaster and The Knight both have holes in their hands, and both have more direct, villainous roles in the story. Additionally, with the notion that the shadow crystals and The Roaring are correlated with 'seeing through reality/one-self', we can interpret that as putting holes in reality. Even Lancer at one point was a villain, and he too loves digging holes. Not all villains are evil forever, but evil ties back to villainy, and villainy makes you immortal.
My other theory stated that The Prophecy is a set of instructions that must be followed to defeat The Devil and The Knight to prevent The Roaring and banish The Angel's Heaven (our world's connection to theirs). I see now that the reason The Weird Route is fulfilling this is because becoming powerful both involves you *unfairly* abusing aspects of the game to gain power (Noelle's powerful spells, thornRing, possession/puppetry and soul influence)... but also involves you becoming a villain, an antagonist whose evil will outrival and destroy The Knight, The Devil, and our connection to our world - and maybe even to Gaster.
Either way, the road to hell is paved with good intentions. The normal ending will not see us winning, because we do not become immortal, and we misunderstand the meaning of the prophecy in lieu of sacrificing Susie. And even then, who can say what the future holds for the world of the lightners and the darkners. Only we can tell, after all.