So, I don't exactly watch WOY, but I came across a Bill Cipher video which had the song, I'm the Bad guy, sung by a female of course. And when I go to find the original song, it's a villainess from Wander over yonder! *Le gasp* AND I ABSOLUTELY LOVE HER, my god. She's about the best female villain cartoons haven't screwed up. But seriously, her name is Lord Dominator, go look her up. You won't be disappointed!
As for the theory....
I always wondered why it look like she had smeared eyeliner under her eyes, as if she was crying, and she had a heart on her shirt/suit similar too it, kinda representing heartbreak and in her song, "I'm not the damsel in distress, I'm not your girlfriend or the frightened princess." Which made me wonder if she was actually a former princess somewhere in the universe. And the meaning of the runny mascara make up an jagged/distraught heart on her chest means heartbreak? Especially in, I'm the bad guy, she crushes a heart-shaped planet.
What if, before she was a villain, she was a princess of a planet/kingdom in the Universe,(a volcanic one because she has lava/pyro like powers) at one time, she actually loved everything about life, cheery and what not, and she ended up falling in love with someone lesser than her, as in a traveler of the universe. But her being a princess, her parents would not allow it. Which she refused other nobles/princes and admirers or well, suitors that her parents sent to her, denying them all. That finally, her parents had grown tired of their daughter's marriage refusals by suitors and had her love executed. Which sent her into a berserker rage, killing her parents off and her entire race. Which is how she became a villain. Now, she doesn't want to conquer the universe, she wants to destroy it so she doesn't have to feel the pain anymore.
Sorry for my jabbering, just a good thought that came to my head. XD