I still think Chara is actually being corrupted by the embodiment of controllable characters in general. And the violent actions you take through them.
I don't think the real Chara was an evil being
the Genocide Chara has been overtaken by the literal embodiment of grinding
and the sense of accomplishment we get from it
They weren't nice, but yeah i don't think they were originally an evil demon
They were a kid who was severely hurt by other humans
enough to make them depressed and nihilistic
I don't think people realize that kids' sense of empathy is not well developed until they're in their early 20's
Kids tend to see the world in black and white
"these people hurt me. Now that I share a body with Asriel, I'm strong enough to hurt them back."
THAT was Chara's reasoning
overly simplistic, and adults would recognize it as extremely problematic, but again. CHILD.
But then in genocide route your actions corrupt them into something unrecognizable
Undertale's main theme is there is no pure evil, everyone is redeemable
so it makes the most sense, considering