It is unconfirmed, but very likely, that Chara didn't fall, he/she (depending on how you see them, I will personally be using 'he'), but, rather, he jumped into the underground because he was abused and bullied by the other humans, so when he saw the chance, he poisoned himself in the hopes of killing two birds with one stone, break the monsters out of the underground and kill the entire human race, including himself, to make the world a better place. In essence, Chara can be seen as an anti-hero, wanting good, but going about it in a villainous way.
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