Conceptually speaking Marie is supposed to represent the very definition of modernism and rebellion and should be analysed as such. I understand people might be put off by her sociopathy, and I do agree, she's a terrible person, especially so in Innocent Rouge, Sakamoto intended for her to be. She's diabolical, unapologetic and downright sadistic. I really don't think the author was trying to glamorise her sociopathy for viewing death as a beautiful thing so much as her just simply being, well, a sociopathic girl who happened to have grown up in the right household to fuel her interests.
She finds mutilation beautiful simply because she loves the adrenaline rush it gives her, and in the sequel, it's obvious that her desire to dismantle the system and punish the aristocracy took priority. Then again, we see her at the beginning of the manga peeking in one the torture room, practicing torture on her dolls and listening on her father's lectures- her whole character can spark a debate of whether it was nurture or nature.