“Greetings, Sans– so.” “This is your job, huh.” “Why do you have such a hard time with it?” “It’s pretty easy.” “Why didn’t you just reap me properly too?” “You’re so incompetent hahaha!”
So, in the First Reaping, Chara was not properly reaped and had thus managed to escape while being corrupted by Death’s (Sans’s) magic. Therefore, with their soul fused by the essence of darkness and Sans’s magic, they have an affinity with Death that allows them to wield Death’s scythe.
At some point, Chara would have grown strong enough to steal one of Sans’s scythes. They take it and reap mortals before their time, which would naturally brew chaos. Sans of course desperately tries to hunt them down in order to take it back, but he is unable to catch them. Eventually, after a mad period of Chara-induced chaos, Chara suddenly appears and drops the scythe by his feet as if it was worth nothing.
“Got bored. Killing people instantly is no fun at all. It’s much more fun… to see them live long enough to become corrupted. Don’t you think?”
And then they disappear with a child-like laugh, leaving Sans stunned and full of dread.
(Whether this was just a random act, simple curiosity, or part of a much greater plot, though, Sans doesn’t yet know. Chara was always so unpredictable.)
He remembered that…[s]he had admitted that no one noticed her because she was a wallflower.
But this night no one could miss her because she stood out from the
crowd. Her gown was midnight blue embroidered with glittering studs and
fitting her to perfection. Her golden hair caught up in a chignon
showing off her nape and neck. She was like a Northern star in the sky
outshining others, and Bill was among those people who was captivated by
her sparkle. ~ Always chp. 10