As noted elsewhere, the Alternative Foreign Theme Songs for the anime are cheerful and happy in spite of this story being about the tragic lives and deaths of people living through The French Revolution, with the French one being the perkiest of all. I can't explain the others, but there is a reason if the French one is so perky: it's the story of a noble soldier that, when the Revolution starts, joins the people against the monarchy. Until you actually watch the series, a Frenchman would consider it the happiest story imaginable.
In the beginning, Montclair was being handled by a maid until she got pricked by said maid on accident. The maid quickly apologizes to Montclair and explains that she wasn't looking. Montclair, instead of being furious, calmly picks up a needle and cheerily says "If you don't need them, I might as well take them!" Cue Gory Discretion Shot.
Rosalie seeing a dead body after visiting Montclair. What if that corpse was the blinded maid?
After all of the events that occurred, Oscar says that it was pitiful for Montclair to search for eternal youth, even if it meant being killed, and André replies that the girls buried under the roses were much more pitiful, implying that, after having the victims killed, Montclair and/or the servants buried the bodies. It's possible that the reason why there were so many roses at the Countess' mansion was to cover up the evidence.