Not all of Kyometsu's crazies are Ranmyaku's doing. Because of his past, Kyometsu has a tendency to lose his marbles. In the first book, he doesn't seem that effected by it. Although his mood flip-flops a lot, he still has a good control over his composure, and his personality is generally arrogant and goofy. As the story progresses it gets darker. His sanity seems more strained. He begins to become less of himself, not necessarily more evil, but empty. But he also retaliates more, going against Ranmyaku's orders. He has a sneaking suspicion that his time is limited, that he won't be granted a high seat of power once Ranmyaku has overthrown the Guardians, and that Ranmyaku plans to bring him down.

Ranmyaku doesn't necessarily torture Kyometsu for the "fun of it" or even when he disobeys. He's often curious of his behavior, and wants to provoke the madness. He takes pride in ruining souls, but Kyometsu is an odd case. Seemingly corrupted already, Ranmyaku knows something is off, and so he is more strict with him than anyone else. Also, he covets Kyometsu's power, although strong himself, there are some things even Ranmyaku is incapable of.

Lyrics and inspiration from Sleeping Sickness by City and Colour.
Kyometsu Amovruor and Lord Ranmyaku (c)meh