I just found the perfect theme and entrance music for his heel turn : Battoutai march
This military march was created to honor the battoutai troops who fought for the Meiji government during Satsuma rebellion.
It was the originally a song contracted out to the French musician Charles Edouard Gabriel Leroux, to praise a special troop made of police swordsmen mainly made of former samurai.
The troop was created specifically to counter the rebels in the Southwest War (the Satsuma rebellion). The rebels were mainly composed of former samurai unhappy with the new Meiji government taking away their way of life and privileges. In close quarters battles, the new government troops composed of former farmers and merchants were hopelessly outmatched by the sword-wielding angry samurai.
The new team played a heroic role in defeating the rebels in bloody sword battles. The song starts off with the following lyrics (loosely translated by me).
I am the Imperial force, my enemy is of the court with no place in heaven nor earth.
The general of the enemy is a hero unmatched through all ages
Following are soldiers all nimble, ferocious, and of unafraid of death
Yet even with courage fit for a demon, those who rebel unforgiven by the heavens
None through the ages have flourished
Till the enemy is vanquished, onward we shall all go
With drawn swords glistening with dew, go forth prepared for death
This song is badass and I love of the enemy is described as better than the singers.