Oh come on! He had just gotten badass!
The sixth episode of Ao no Exorcist: Kyoto Fujouou-hen (Blue Exorcist: Kyoto Impure King Arc) proceeds with Shura urging Yukio to continue reading since she and Rin want to hear what Bon’s father has written for them, while Mamushi goes off with Todo to where the eyes were originally sealed. Along the way she has a flashback to where she played with Shima’s brothers and was scolded by her father, only for Bon’s father to come up and ask her to return. The happy memories quickly come to an end as they approach the temple, only to find that the sacred flame is lit despite it being abandoned to all appearances.
Back with the flashback, we see that the sword Rin’s using to hold his power once contained a fire demon that used its flames to defeat the Impure King. Shima’s father basically then tries to attack Rin’s father, but in the end he doesn’t actually manage to hit him. Instead, it turns out that he was injured the entire time with wounds that should have killed him. Bon’s father basically decides to treat his injuries and they have a little talk.
Fujimoto antagonizes him by stating he believes that his boss intends to use it to kill a kid, which we know to be a lie, so Bon’s father tells him to get lost. Things happen and they stumble upon a room where the sick are being prayed over, which Fujimoto interrupts since the praying isn’t helping and they need to pull the toxins out of their bodies to heal them. He then goes around and helps them, including Bon’s mother.
But he still wants the sword and they can’t let him escape exactly, so they come after him. Bon’s father decides to help him escape since his father was willing to forsake lives for the sake of their sect’s rules and then tosses him the sword, revealing that he knew all along that the sword was empty inside. This act basically solidified them as friends in Fujimoto’s book, and a few months later Blue Night happened and Bon’s father learned the true secret of their sect as his father died.
Back with Mamushi, she and Todo find the secret chamber in the temple and he manages to weasel the eye from her. This proves to be a mistake since he uses the eye to reawaken the corpse of the Impure King, revealing he knew their sect’s secret all along and was now a demon. Bon’s father shows up to stop her from being killed, also revealing that he had inherited Karura from his father and the price of this contract was secrecy.
Unfortunately, their efforts to stop Todo amount nothing since he manages to awaken the Impure King as Juzo arrives in time to see the spectacular CG fungus. So Bon’s father passes Mamushi off to him so that she can inform everyone of what’s happened, while also dealing with the fact that since the secret has gotten out Karura’s pact to serve the head priests of the Myoda has ended. Thankfully, he made another pact and has a technique to confine the Impure King.
That’s when Todo pops back up and shanks him in the throat, revealing he did all this and doomed countless people so that he could get his hands on Karura. He then proceeds to eat the demon, forcing it to possess him, and it de-ages him to his youth as a bonus. The episode ends with the letter to Rin stating that he wants him to use the sword to kill the Impure King.
So, as I pegged earlier, Mamushi basically had her faith shaken in Bon’s father and that led to her being a rube and falling for Todo’s sweet-talk until he screwed her over. In the end, she has to be saved by the man she mistrusted the most from the man she put her faith in entirely. It’s beautifully ironic because the demon familiar that Bon’s father uses feeds off the mistrust and arranged things specifically to where those of Fukaku’s line would be mistrusted in exchange for his power, meaning we have a good-old fashioned deal with a demon here.
Naturally, they all get screwed over by Todo, and Bon’s harsh words from before are mirroring Rin’s in that they are among the last things they’ll get to say to their father’s.