Well, Miyabi's motivations must still be dominate the world outside the island with a vampire supremacy. I guess he is just really patient overall not to just go against his word towards Akira (albeit at the same time being a treacherous rapist bastard who previously used of very dirty means to win his conflicts and to progress with his plans). Yeah, I don't get where his characterization is going exactly, but after the whole "shounen spirit of protecting everyone and growing stronger" transformation on Akira's characterization, I stopped asking these questions and just enjoyed the ride.