...and journeyed to New Home. (there also appears to be a missing history book volume in between these two volumes)
Asgore himself had also changed his mind about humans killing them all once they left the barrier.
Why would he change his mind after all those millenia?
Because he had the "future of humans and monsters".
Chara fell when the monsters were still in the Ruins, and was heavily implied to make them unafraid of humans any longer after millenia, brave enough now to populate the rest of the cavern rather than staying as far away from the barrier as possible, and heavily implied to have changed Asgore's mind about humans too.
By necessity, we’re going to have to toe the line of HOPE a little here, since Chara has never stated the reason for this outright in their entire life.
This statement comes right after “I know why Chara climbed the mountain. It wasn’t for a very happy reason.” with no pause; Chara’s reason for climbing a mountain people disappear from, for wanting to “erase themselves from existence”, is related to their reason for hating humanity.
But, unlike their reason for climbing the mountain...
they refuse to talk about this, even with Asriel. Suicide is already a very private topic, which they were willing to breach with Asriel, so there is something even more private going on they explicitly don’t want him to know about.
And no, it’s not because they killed someone.
These are their starting stats. Their Level of Violence is 1, correlating with 0 Execution Points, which go up any time the Frisk-Chara Combo Pack kills someone.
Was their reason for hating humanity because humanity is objectively the worst, ever?
Nah, Asriel knows this already. It seems they’ve probably talked about this. At length.
So the reason they hated humanity is tied to something specific, private enough that Asriel can’t know, not murder, and big enough that they wanted to erase themselves.
In relation to this, I’d like to call attention to an indisputable fact.
The distance between the village and what they see as Death Mountain is about the distance of the underground, which we can walk Frisk through in a day or two. Given its track record, people are probably forbidden from going there, so they’re not hitching a ride there and since they're children (the coffins all being the same size as Chara's coffin and Frisk's sprite barely fitting into one) they're not driving either.
The children who flung themselves into the Underground probably had to walk there on their own. They probably all came from the same settlement. Eight children who came from this village all tried to "erase themselves from existence" in a relatively short span of time.
Also, by the time they get Underground, they have something in common.
Context: after you reset and she guesses your favorite flavor.
Context: Asgore completely unsurprised to hear you can come back from the dead and have many times. He fought the other children, and it seems he’s heard this story before.
The children who climbed the mountain and came from the same village as Chara somehow had the ability to SAVE and RESET on their journey. But they gave up at the end, and without determination, they died for real, and their ability to save and reset was passed to someone else.
We know that only one being can hold save/load powers at a time, since Flowey's are taken away when a being with more determination arrives. For some reason, this individual has been the fallen children on many occasions if not all of them, and that's no coincidence. There is a reason for that.
I believe Chara also had that power. Frisk’s SAVE file is in their name.
And there’s this question that Flowey asks them at the end of Pacifist.
He’s asking them if the power to save and reset was one they were fighting to stop. Given that they use this power along with Frisk even in Pacifist (to, at the very least, keep them both alive), I he may be referring to their efforts in life. Maybe they believed their death would erase this power. They likely lost it upon committing suicide, having lost their will to live/determination.
And how does this relate to their possible reason for hating humanity, that they could never tell Asriel about?
Even after he says Chara was fighting to stop the resets, he makes this accusation. I think this is what they might have been afraid of. (although this is a fair accusation, because starting the game up could only indicate that there is an intention to reset and play again, and he has picked up on that and only confronts them if it is started again)
But Chara seems to hate the power to reset's potential for using it to dodging responsibility. Your attempt to abuse it completely sets them off.
They have an overwhelming drive to hold you responsible for the actions you take, to prevent you from getting away with outright abusing the power for fun without consequences.
It's possible that their secret power to reset is linked to their reason for hating humanity, which means it, too, must remain secret. That the humans in the village tried to abuse their power, and the humans were never held accountable for it, and that other children who could reset were driven up the mountain decades later for the same reason.
It would also explain some strange actions taken by Chara during the trip outside the barrier.
We know Chara has some kind of real fixation on these flowers - they have drawings they made of the golden flowers next to their bed. But I don't believe they made this request, and carried their body over just to see the flowers.
Was it to force an attack on Asriel? Hell no. Chara already has control over his body before they even pick up their body. And he's already agreed to take six souls. If they had any idea that Asriel would have "betrayed" them, they would not have trusted him with the reason they climbed they mountain, the locket, and their soul.
I think the beliefs of the villagers and their subsequent attack was an accident that the combined intellectual "might" of two 12~ year olds couldn't have predicted. Plus...
It seems that the monster-human combination with one soul is a little frightening to look at. The God of Hyperdeath has enough souls to generate an image to his liking, but with one soul, his appearance is "very unsettling" and described by even the monsters as a "horrible beast". The villagers might have assumed he must have killed the child he was holding because of his appearance.
And if Chara really wanted the villagers to attack, they could have mutilated their own body in front of them, faked an attack on their corpse; all they did was carry it to the golden flowers. The image was likely similar to a parent carrying their sleeping child to bed.
But I do think Chara wanted the villagers to see their dead body.
Chara, for reasons unknown to us, has an extreme fixation on holding people accountable and responsible for their actions. Their reason for hating humanity - I believe this originated from the villagers, who did something so awful that Chara thought an appropriate punishment for them was death.
They want the villagers to see their dead body; to hold them responsible for driving them up the mountain, to show them the reason they were about to die. And if Chara had the power to reset as long as they had determination, that the villagers abused, Chara’s death even being a possibility is an indication to everyone that they lost the will to live.
I think they're trying to say...
And I don't think their eventual plan was to restart a war on humanity - they seemed to truly want to break the barrier.
That said, would they have been altogether that fussed if monsters managed to defeat the hated humans, and regain control over the earth, after being slaughtered and trapped underground? Probably not. Many monsters themselves, during the events of Undertale, are "so hype for the destruction of humanity", after all.
And no, I don't believe "premeditated murder is A-okay as long as one's backstory is tragic" (which no one has ever claimed), but this way of thinking is also analogous to the monsters believing it is alright to kill seven children to break the barrier, despite monsters souls supposedly being made of love and compassion; it is a flawed "ends justify the means" sort of thinking and Undertale's message is that there is no right reason to take a life.
Chara also refers to the plan to break the barrier as "our plan", not "my plan".
Their plan was to "just" take six souls - this was as far as the plan was Asriel-approved and could be called "our plan", which was...
If there had been any ulterior motive besides that they would have said "my plan", although Asriel realizes in hindsight that breaking the barrier with murdered souls would mean having to battle the vengeful humans beyond, as his father was on his way to doing, after declaring war on them.
It’s worth mentioning that the narrator has a little moment when the barrier is broken:
It's one of the very few moments when the narration voice goes nonstandard in the game, and it even matches Chara's way of speaking at the end of the Lethal Limbo (but it's not as slow and spaced out as their reaction to Asriel's phonecall). The breaking of the barrier spawns an emotional reaction in them.
They may have even thought breaking the barrier was their purpose, as someone who was "the future of monsters and humans".
Remember what Asgore tells them on their deathbed?
They've probably heard this line before. This is the angle Asgore pleads to them from.
Now, as a singular human with a singular human soul, knowing monsters can absorb human souls to exit the barrier that's trapped them for millenia, how does one become the "future of monsters and humans"? How would you, as a suicidal child, have interpreted that line?
How would you have fulfilled your dad's wishes for you to become "the future of humans and monsters" knowing what resources were available to you: your one human soul, your boss monster pal...and your knowledge of where to find 6 of some very bad humans' souls?
We know they held this line very close to their heart, because...
...even after death, they remember it.
There are clues scattered around New Home, and in the behavior of living characters, that indicate that Chara had a mutual love of their adoptive family.
Toriel obviously cared for Chara very much. She took them with her to the Ruins to give them a proper burial when they died...
she took the seeds of their favorite flower to plant over their grave...