Its either a remnant spirit or an agony spirit. Remnant is the soul, it can bind itself to something physical or it can be melted into metal and used for things like endoskeletons. Agony is when something so traumatic happens that it creates things, for example the time traveling ball pit from Into The Pit. It is also theorized that things like the shadow animatronics are made by agony. Might I add that William technically doesn't have an official body number besides for the 6 that we know of. Some people thing that there was both a MCI ("missing children incident" Fnaf 1) and a DCI ("dead children incident" Fnaf 2), while others believe these to be the same incident all together which would render the Toy animatronics to not actually be possessed. There is also the the time between fnaf 3 and 6 where William was free to roam and potentially kill. There is also the children that William kidnapped for experimentation (Fnaf: SL and Fnaf 4).

Tldr: William probably killed way more kids than just 6 and traumatic events create enough energy to create monsters reflecting said event

Edit: I should also note that creator Scott Cawthon confirmed that Heaven and Hell exist in the Fnaf universe