There was once a child. Their name was Chara, they were once a child full hope and love. They were so naïve of the fate they were forced to live.
The village they lived in had a cult, they would chose a child to behold the evilness that humans had inside of them.
The people spat, beat, tortured and blamed Chara for every terrible deed that had every happened in the world, they chose to slowly tear the naïve child apart, it took a huge toll on Chara, giving them the terrifying look on humans.
Because of the people, Chara created a new personality, overshadowing the once naive and kind child.
Soon Chara couldn't take it anymore.
They tried to committed suicide on Mt. Ebbot.
Instead of dying, Chara found themselves in a much kinder world before the one they once lived in. In the world of the monsters, they found themselves with a family, a person they could call friend and love.
They were happy, until the monsters spoke of destroying the barrier separating the monster world from the human world, the deeds of the people who did so much wrong began to tumble onto Chara once more, their world had gone dark again.
They had come up of a plan with their best friend Asriel, but for Chara in secret this plan was to eliminate the people who did Chara so much wrong.
They went through with this terrible plan.
In the end, the small side of Chara, that is the part of Chara that always loved Asriel with all of their heart, broke free, they waited and waited.
Then, Frisk fell into the underground before they exit the Ruins, Chara asked if they could come with them, of course, because of Frisk's kind nature, they said yes.
In the end Chara finally for the last time saw their family in the eyes of Frisk, the joy they felt and they finally saw their brother that they loved with all of their heart, they finally with their new family got to the surface.
" Thank you " and with that Chara disappeared finally happy.
I just wanted to give a back story of Chara Dreemur, I was wondering why they hated humanity then it clicked. Also this is for the pacifist route, I maybe will be doing the Genocide version of this.
Undertale Comic - Scapegoat [Chara's Beginnings] this where this headcannon was born at.