That is a shitty reason!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
The reason Miyabi Fujisaki (a.k.a. Alucard) broke his promise is sadly simple. Once he knew the Rosario couldn't control Alucard, he no longer saw his deal with Akua as benefitting him to be kept. It's like some old westerns: the bad guy cuts a deal with a family or a nemesis to avoid getting beaten. Once the threat to the bad guy is gone, a double-cross is "par for course". Also, Miyabi, as Alucard's clone, retained his ruthless cruelty and arrogant wish to be powerful. After all, the original Alucard consumed multiple monsters to become stronger, but lost human like form in the process.
You mean, "Why did he break his promise"? And it should be obvious, because that's what monsters/people (people who are monsters) from Fairy Tale do.