Well, from Origins, Hawkmoth seemed to imbue the butterfly with dark energy, and then when that possesses a host, it takes shape as the supervillain persona.
What I think, because Hawkmoth is evil and have evil thought's and that's why the Akuma is full darkness.
Since the Butterfly is supposed to make superheroes out of normal people and not supervillains, I think it was originally meant to use positive emotions as a means of creating Akuma. Hawkmoth's dark purposes have instead corrupted its function.