Without the messages contained in the game, there is nothing to support it and everything collapses. The main reason to play this game is for the message of "peace", and without this as a theme, there is nowhere for the game to stand on its own.
Fallout 1, 2, and New Vegas allow players the freedom to kill whoever they please. The only consequences for doing so are that you lose the ability to complete some missions, and the other factions will see you as an enemy, and will try to bring you down. The game will remind you if you have killed anyone notable, and may even tell you that you are "bad" for your actions. There is no need to remind players of their actions, as they may already feel guilt. Characters that are important to the story will change the game if you choose to kill them, and you will be punished for it. Some characters will be upset about it, others will be glad, and some will even ask you why you killed them in the first place.
Undertale and Fallout are completely different in gameplay, but both are RPGs, and both punish you for going back on your decisions. Undertale makes this punishment worse by shaming you for choosing to do anything differently, and makes you feel sympathetic for the monsters that you attack. The villainization of the player also makes the game less entertaining for some people who simply want to play the game for either fun or story. In addition, it is obvious what happens when you kill a monster, so you cannot avoid the guilt that the game forces you to feel about your actions. Undertale is a game that is entirely based on guilt tripping you when you play neutral or genocide runs. The game even removes certain features that you paid for, just so it can make the game "interesting." The game also blames you for simply choosing a specific route, even if it gave you the option to do so in the first place.
Fallout does not care about the people you choose to kill, and even if they are important, the game will still run. However, Undertale will directly call out your bad choices if you do not play the game the way it was intended to be played, and try to make you feel guilty for your decisions. The game even hints at getting a different ending by killing everyone, which is something players might do after they beat the game and have gained the "True Pacifist" ending. Undertale villainizes you when you play the game differently from the "True Pacifist" route. Giving players options and telling them that they won't like the consequences is like God giving Adam and Eve the power of knowledge, and still punishing them for using it. As if he expected them to know what they were doing when he told them not to do something they didn't even know existed.
Undertale has the option to play this "genocide" route, but tells you not to do it because your choices have consequences. Even though the game expects you to complete this route anyways, it still tries to make you feel guilty when you complete it by telling you that you've done wrong. It is clear that Toby Fox and his game have specific ideals and that they want to tell a story, even if it means shaming and demonizing the player for making their own decisions.
While Undertale provides many different routes when playing the game, only the Neutral and Pacifist routes were fully developed. The Genocide route was incomplete by the end of development, and only partially finished, and Hard Mode was never finished and was only included up until the Toriel fight.
It feels that you think that the gameplay and flow of the game is boring. Perhaps it is the repetitive gameplay or the lack of variety of gameplay that makes you believe the game is boring.
Frisk is not only a poorly made character, but a poorly made avatar as well. The mute avatar type has its limitations, especially in games where more than a few lines can be said. Claude Speed, Gordon Freeman, Link, Mario and Luigi (M&L), and Mario from Paper Mario are silent avatars that, although they do not speak directly, speak through their actions or with simple yes/no questions, allowing the player to make their own decisions and answers.Another example is Ness from the game EarthBound. Ness is a quiet avatar, and the game itself takes advantage of this by making fun of the quiet avatar trope. Ness is silent throughout most of the game, so when Porky is devastated and asks Ness if he will be his friend, Ness simply stays quiet, making Porky think the two are not friends. This ultimately breaks Porky, and makes him join Giygas with the intention of seeking revenge against the person who he thought should have been with him forever, but instead just remained silent. There are inconsistent limitations on what the player can and can not say as Frisk, which ultimately causes the failure of the character as a silent avatar. Having inconsistent speaking limitations makes them feel quite spontaneous, as the player can not choose the right thing to say in the right situation. The situation with Asgore is a great example, as you can decide to speak to Asgore peacefully, or try to attack him with your weapon, but either way the game makes him the victim without giving the player any real freedom to change the outcome.
The fact that the game is so narratively manipulative and justifies the behaviour of its characters, be that toxic habits or cruel actions, allows for its fandom to become one of the worst on the internet. This is mostly due to it being a game that often attracts a younger playerbase, and they are sadly easy to persuade, influence or manipulate. When the game justifies harmful acts, this is a toxic habit for people who play the game to start to believe.
