Chara is the gamer instinct
That's right, the implication at the end of the game that Chara makes is that they are the feeling of mindless progress that so many video game players grind for. The endless increasing of numbers for no other sake than the increase itself. If you've ever played a mindless dungeon crawler and leveled up way beyond what was needed to progress, simply because you enjoyed the feeling of winning battles and seeing your level increase, that was Chara, The embodiment of the player's desire for power, stats, and destruction (EXP, LV, HP)
Toby didn't handle this great in undertale, (though I still think the meta-narrative attempt is cool, I just think they should have been two separate characters) trying to combine Chara the in-universe character, with Chara the demon of mindless video game progression, left some misunderstandings between the two.
The implication Chara the demon is making when you disagree with them and they say "Since when were you the one in control here", is that you are driven by your lustful need to increase your stats, to the point that you would destroy the entire world, the game itself, if it meant you would gain a level, even if you outwardly deny it.
Thus why when you agree with them they say "you are a great partner", and "let's destroy this world and move on to the next", the meta-narrative implication here is that you, the player, will host Chara in whatever video game you play next, repeating the cycle of attempting to kill every monster, exhaust every option, gain as many levels as possible, etc.
The "destroy the world (game)" thing is also somewhat symbolic of what happens to the relationship between the player and the game when they interact with a video game in this manner; because when you interact with a video game in a completionist manner, when you uncover every secret, gain every level, see every little detail, the game itself ceases to function as an illusion, it becomes nothing more than lines of codes, the characters become nothing more than sets of dialogue, all of which you've seen and experienced. The game ceases to be. Flowey alludes to this as well in some of his dialogue. When you engage with a video game in the manner Flowey and Chara talk about, you literally consume it until there is nothing left to consume, and then you move on to the next game to do the same thing.
I do assume that Deltarune is going to attempt to make a neo-version of this point, maybe with some yoko taro-style twist on top of it where re-experiencing things adds new context to the consumption of the media, but I'm just speculating about that.
