Montparnasse reminds me of the Artful Dodger, in a way. Their crimes are fuelled by their desire to be a part of the upper class, but they live in a society that will never let them do that. The system failed them both before they ever had a choice in anything. The system failed to give them a real sense of right and wrong, or even to feed them. Remember how Montparnasse was a gamin, and the freedom of being a gamin is temporary? Montparnasse has grown up now, or at least grown up enough that he won't be able to sustain himself by running around happily like he used to. So, what are his options? Become a labourer, pretty much. His beauty and youth are the only things left of the innocence of childhood, and becoming a labourer would ruin those. The system has failed him, and the only way for him to avoid his lot in life is to become a criminal, to ignore the system.
There's a lot of irony in him killing to be seen as a gentleman, but I feel like Montparnasse wanting to be well dressed isn't just him wanting to be fashionable. It's his longing for a world he's never known and never will know. He doesn't want to have to kill to eat, he wants to be carefree and to have someone else feed him. He wants a simple and easy life, and, even while breaking the rules of the system being a criminal, there's no way for him to get that life. So, he does what he can to imitate it, as does Dodger.
And Montparnasse fanon is horrible, Relly, I feel your pain.