I know Undertale makes Holla Forums's panties bunch but I want to talk about the Genocide route, as well ramble a bit my personal interpretation of the game, which runs counter to most other people who've played it.
Now you might've missed it being too wrapped up in the joyful cheeriness of the game (Or disgusted at just how Tumblr it is depending on your taste) but Undertale is a game that outright hates the player, especially those that tend to feel real world emotions towards fictional characters (AKA waifufags). The game starts off by telling you how to spare people, and then it outright tells you that some monsters need to be hit a couple times before it lets you spare them. Then you get to Toriel (Who is arguably the biggest target of waifufaggotry in the game). What happens if you do what it tells you? You suddenly do a shitload of damage out of nowhere and kill her. Why? Becuase fuck you, thats why. You're a horrible person for doing what the game told you to do to try and spare her. You should've just trusted this monster you barely know that she wasnt ACTUALLY trying to kill you. Just harm you. You need to wait and do nothing. Don't even think about reloading a save, or we'll know.
To make matters worse, the only character that even knows you exist is Flowey. Throughout the game, Flowey's job is to remind the player how shitty of a person they are and are probably like two seconds away from saying "fuck it" and murdering someone. Theres even a joke about "someone that we dont know who exists" (The player) to remind them that no matter what feelings they feel towards the characters, its not mutual. What happens when you DO succeed, and save everyone except the ONE person who acknowledges your existance? You're told to fuck off. You're presence can only make it worse for everyone. Sometimes when you beat a game, the main menu changes, usually to something that makes the player feel good about what they've done. Not Undertale. The previously upbeat music is replaced with empty mechanical noises and all the characters that were previously there are gone, leaving only black to remind the player how alone they are. The No Mercy endings are even worse (Including the one you HAVE to get first before the pacifist ending) becuase they highlight how if you even kill one person, the underground will be all the worse for your presence.
Genocide on the other hand, is different. At no point does it try and trick you into feeling bad or like an idiot (That is, as long as you CONTINUE doing genocide rather than going back on it like a pansy). It even makes a point to outright say you arent doing what you are doing is evil, but just the natural evolution of what you, as a player, are meant to do: push a game to its very limit. Its also easy to miss but the only time Frisk ever smiles or shows any emotion at all is during genocide whenever a random battle starts. The ending is also the best, you are faced with Chara, only their name ISN'T Chara. TobyFox has said that the first human's name is whatever the player's is. Theres a good reason for this too: while Frisk is a vessel for the player, Chara is a reflection of them, mostly of their desire to be victorious over games. Chara is that part of you that pressed you on despite how grindy Genocide is and with how much of a difficulty spike Genocide Undyne and Sans are. It ends with you and Chara "erasing the world and moving onto the next" (Basically uninstalling the game and moving onto whatever game you have next), and if you decide against doing so, it reminds you that doing so was the whole point of your journey. When Chara says "Since when were you the one in control" it wasnt just meant as a spooky "I am more powerful than you" it was meant as "If you were in control of your behavior, you wouldnt be here right now"
tl;dr Undertale's subtle underlying message is that the player is a disingenuous monster who at best will fawn over fictional characters and at worse will just fuck everything up.
Yeah I know I fucked up accidentally posting as a reply to a thread