Kaibutsu was clearly insane after suffering almost all his life. Due to his insanity, he is very aggressive and impulsive, but also has some cunning aspects.Though he seemed monstrous and evil, it was all result of the experiments done to him by humans. Because of it, he hated humans with passion and thought they betrayed him.He attacks and kills harmless humans, but knows that humans are dangerous and stays away from those kind of humans.He believes humans and their dogs are cruel and that he is not. Indeed, Kaibutsu mentions that he does not want to make his opponents suffer longer than they have to. The only one Kaibutsu ever saw as his friend was Jerome and that is why he didn't want to kill him.

Japanese voice for reference. https://youtu.be/8KSQ925B2wg?t=6m25s (This is a voice that will most likely be layered in editing)