Bunny is obviously unhappy, and expressing himself the only way he knows how. He sees her as a threat, and is marking his territory accordingly. How did they introduce them? Did they do it gradually, like one would with two cats, or just bring her home and expect things to be fine? Is the kitten allowed everywhere? If she has been around his litterbox, he could not want to use it, and if it where she can see it and get to him, he knows he is vulnerable when using it.