That's okay, there are plenty of ways to skin a Frenchman
Early marketing materials for Assassin's Creed Unity have largely included guillotines, headless bodies, and severed heads on pikes. It makes sense; the clash of classes that was the French Revolution was an awfully bloody affair, and decapitations played a major role.
However, when Unity hits shelves, players won't actually be able to take that aspect of the revolution into their own hands. When asked if decapitations are available in gameplay during a recent Q&A session over Twitter, protagonist Arno Dorian's voice actor Dan Jeannotte commented "The marketing guys are telling me 'no, no it is not possible.'"
While Ubisoft made the unsurprising call to not let you cut the head off of every guard you come across, there's sure to be plenty of decapitations and severed heads during cut scenes and scattered about the environment. Besides, just because this method's off the table doesn't mean there aren't a ton of other violent means at your disposal.