Asriel had bad history with humanity, and Chara started to despise them because of it.

Asriel sacrificed his SOUL as an attempt for Chara to cross the Barrier and find a peaceful way to free monsterkind. But at the first sign of hostility, Chara tried to attack (only stopped by Asriel), getting both of them killed.

When Chara turned into a flower, instead of losing any emotions, Chara lost control of theirs, making them overly sensitive and unable to reach the limits of their power (unable to kill anyone).

Chara would call themselves "The Devil" on the true pacifist final boss.