Rose of Versailles/WMG

Rosalie was the one who shot Andr Ã©.
She certainly had a motive (she was rather fond of Oscar, after all), she was close enough to have done so, and thanks to Oscar, she had access to firearms and knew how to use them...

Or maybe I just like the idea of a Badass Rosalie with a gun.

Oscar had a relationship with Ben Franklin.
It's known he had a way with the ladies and he was in Paris shortly before the Revolution started. And it'd certainly explain why she turned revolutionary at the end. Could be a perfectly platonic relationship. Ok, I'll admit I'm just a fan of Franklin. Hey, better than John Adams, eh?
 * Actually, it was Madame Dubarry that did. Think about it. She's notorious for being promiscuous in-series, and would like a rich man she could get power and recognition from.