In some ancient civilizations the Wolf was a symbol of the underworld. In Egypt, the guardian of the dead was Anubis who was a wolf-headed god. The jackal portrayed as black was the symbol of both death and regeneration.
William is also the one who hid inside the suits and kidnapped and killed children, leading to the assumption that William Afton is The Purple Guy of FNaF 2 and 3 (he is killed at the end of the book when Charlie sets off the spring locks in his suit).
There is no evidence that he actually dies in the Night 4 Phone Call. My theory, him being the Purple Guy, is that he was recording the message, was attacked, and retreated to the safe room in the pizzeria where he was killed.