Is that how a real final villain is supposed to act?

Its a disgusting and low move from someone in a story where everyone else usually put their bodies on the line for their dreams. Even someone as Bellamy had principles.
Blackbeards principles and morals are non-existent unless the situation demand any and solely for him to gain some advantage from it. Whatever narrative and advantage he can get to backstab, manipulate and scheme, he will do it.
Absolutely the perfect counterpart to Luffy.