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).
Purple Guy also killed all of the animatronics without hesitation, he was brave, just like Fritz. Yet, Purple Guy shies away at the Puppet. In FNAF 2, no matter how good you are at the game, you will never master the Puppet. He completely depends on how occupied you are, you have no control.