why shouldn't you wake up a sleepwalker
There used to be a myth that waking sleepwalkers would cause their soul to become lost. That myth has fallen out of fashion. Now, the main reason you don't wake a sleepwalker is because they may become confused and lash out upon waking.
The reason you don't wake a sleep walker is because you don't know how they will react when woken. Some people are frightened and disoriented, while other can be aggressive and violent.