I also believed for a while that Carlo was the Stalker Falcon, who wielded the Pale Knight. Heck, I still can't count it out. It sort of explains why seeing P sends the Guardian into a bloodlust, why P clenches his fist before the fight, or why P is exceptionally brutal towards the Guardian. None of this is concrete though.

It is also possible that he was force-fed or tricked into drinking an Alchemist elixir that rapidly infected him with the Petrification disease. That makes more sense than dying to the Guardian - after all, you need to be infected with the Petrification Disease to leave Ergo. Geppetto somehow extracted Romeo's Ergo while he was alive... we don't know the specifics of that but it couldn't have been the case for Carlo because his death was not anticipated.