Chara never called themselves "A demon". They simply said: "Greetings. My name is Chara. your power awakened me from death. my "human soul" my "DETERMINATION", those were not mine, but YOURS. at first, I was so confused, we had failed. but now I know my true purpose. Let us erase this pointless world(From when we killed everyone, they want us to make a new one) and move on to the next."