Kyoko Otonashi's Resignation Letter

The body of an adult woman is found in a park pond. The cause of death, the motive, and even the killer's identity have all been established, yet there is still a mystery for the Forgetful Detective to solve... The pond is only five feet deep at its deepest point, so the body at the bottom is visible through the water. Why did the killer sink the person they had murdered in this pond? The astonishing reason is about to be revealed!