"At least we're better than those sickos that stand around and WATCH it happen...
Those pathetic people that want to see it, but are too weak to do it themselves.
I bet someone like that's watching right now, aren't they...?" - Flowey
It's interesting that the genocide playthrough take up 1/3 of the game and the story and yet it's an aspect of the story where a quite significant proportions of fans of the game refused to play, take pride in not playing it and even questions the morality, ethics and characters of people who do experience that aspect of the story.
In a way it's a testament to Toby Fox as a storyteller to elicit this kind of reaction. By creating such memorable characters where even random monster people encounter have distinct personality, why would anyone wants to go around killing them? You have to be an amoral psychopath to go through that people will understandably argue.
Also Toby Fox has mastered the integration of interactive elements of video games to the plot of the game. Saving and loading the game isn't just a video game mechanics but an intrinsic part of the story and hence maximise the immersion of having the video gamer self-insert themselves in the protagonist "Frisk "and give players the illusion that you are the protagonist not just a passive observer or an actor playing a role as alot of the video gamey elements that could potentially break the suspension of disbelief is integrated in this universe. Hence Undertale creates the illusion that you are Frisk with the power of determination to moulded the world in your vision. Any negative actions that Frisk does hit home and fills the player with guilt as you are the person creating harm not just a fictional character.
So when the reformed Flowey pleads with Frisk/the player to leave the universe alone and let the true-pacifist happy ending stand instead of resetting the universe and playing the game and erasing the triumphs of all the characters of the game. It's quite understandable that people left the game as it is and not touch the game again and not be a completionist and explore all the stories that could be told in this game. Especially when the motivation of the villain of the game is essentially a completionist who dispassionately seek to discover everything in the world. Why would you want to emulate the villain of the game?
So is that the end of the argument that the "correct" way of playing the game is to just play the game until you get the pacifist ending and then just quit leaving the game installed in the computer but untouched and imagining all the characters living happily ever after?
My answer is no and in fact this aspect reveals how "self-insertion" by the gamers is a limitation of story-telling of video games as a medium. In a way I see video games as players being an actor in a choose-your-own adventure story. You are given some degrees of agency to shape the story like most actors do but ultimately you are following the script of the author/game programmer.
Now I'm not necessarily saying that self-insertion is wrong and in fact it is a strength that gives video game it's unique qualities and in large part it is part of the reason of video game popularity. People self-insert themselves as an idealised heroic version of themselves in fiction and when they finish the story they feel they have shared the accomplishment along with the fictional character.
Although this is a strength of video game it is also a weakness. It's the reason why video games are singled out in terms of controversy. After all, when people see people "playing" video game characters doing immoral violence, they are not just seeing a character doing violence but seeing the player themselves as an active participant in violence which does scare alot of people.
However, there is an alternative to "self-insertion" and the answer is in the name of the genre of Undertale itself which is "Role Playing". That you aren't playing an idealised version of yourself and playing a character with similar ethical standards as you. You are playing a character that you created with its own personalities, agenda that still makes sense from the universe the author creates.
The question whether people playing the genocide route are amoral psychopath could be rephrased and asked are actors who play villains in movies and not only that but enjoys playing villains in the movies, are they amoral psychopath? Is Heath Ledger a psychopath for playing the joker in Batman? For people who seriously morally object to playing the genocide route of Undertale, you have to ask yourself what makes you different to the moral crusaders like Jack Thompson who believes video games makes people violent because it's interactivity of violent behavior in video games. The inability of people to see the interactivity as anything beyond self-insertion is the reason why video game controversy exists and gamer themselves should absolutely resist this mentality as you are falling in the trapped of people who are essentially anti-video games.
The controversy of people playing the genocide route and people being question about why they want to see well-crafted characters get cruelly killed off now seems absurd if viewed from a "role playing" point of view. As the answer is that it is good storytelling. I mean it's the same reason why people write books, films, TV shows that have well-made characters be killed off.
The genocide ending and the subsequent soul-less pacifist is great storytelling. Imagine a person with the ability to time travel to an earlier version of themselves and redo events in their lives and even avoid deaths. At first they go and do things to make the world right and to correct their own mistakes to ensure that everyone they care about get their "happy ending". However eventually their own curiosity and boredom with the power where they feel they discovered everything about the world they know within the constraints of morality. They decided to start killing people out of curiosity to see what would happen rationalising that they could always go back in time and reverse their choices. After all, from the perspective of the character, is murder really bad when murder is easily reversible with time travel and you can always undo every consequence of your choice whether it is good or bad. However, a person can't just indiscriminately killing people without affecting themselves permanently. You can't just decide to commit genocide and then just reverse with a click of a finger as no one can make that decision to do these great evils without changing yourself and turning yourself into a villain. This is reflected in-game by having Chara taking control over the body of Frisk and decided to kill Asgore and Flowey and then the rest of the monster universe without any input from the player. Once you made the step to be the villain you can no longer have the morality to reset things and make things right and have no control as a person has to become the devil or "Chara" to make that decision in the first place and that moral centre that you had is no longer in control as you unleashed your own dark side (which is represented in game as Chara). This is shown where even attempts of redemption in doing a pacifist run after a genocide playthrough resulted in the destruction of the world as once you cross the moral event horizon you are no longer in control but your own inner dark side is.
To me that is excellent storytelling and there's a certain poetry to have the protagonist becoming the villain that they tried to stop in the initial "Pacifist" playthrough. Also the genocide playthrough reveals character insight to many characters of the game including the primary antagonist. One of the message of the game is that none of the characters are "purely evil" and they all did things with good intentions. It's their good side of their nature that comes to the fore when faced with a villain as powerful as the "Chara" corrupted Frisk and that is good storytelling. Ultimately "self-insertion" as much as it is a unique positive aspect to video game can be a pitfall that limits how much a person can get out of the medium and limits the potential story they can enjoy if people are unable to divorce themselves from the protagonist they are playing as this prevents stories that explores the dark side of humanity from being seriously appreciated.
Does this mean that in my playthrough I did the genocide ending after I finish the pacifist ending? The answer is no. After seeing the plea from Flowey to leave the game as it is and the dark music over the resetting the game option I didn't have the heart to go through with the genocide playthrough. After all not every person is capable of being an actor to be a villain in a stage or movie and will have a lot of trouble doing villainous act even in a scripted event with no real world consequences. However, I see this as my own weakness and not a strength. That my own inability to roleplay has limited my ability to fully enjoy video game as a medium of storytelling. I only have to admire people who do have the strength to go through with that and in that case I agree with Flowey that people who do the genocide run are getting more out of the game then people who are unable to go through it or is only able to watch it. I wish I had the strength to go through with it. So for all those people who are refused to play the genocide playthrough of the game, that is perfectly fine but please respect the people who are able to explore all aspects of storytelling in video games by role-playing instead of self-inserting themselves and their own personal moral standards on the characters they are playing.