Remember how the killer Used the Spring Bonnie to kill the 5 missing Children?
William is the One in Purple and The Person he is helping putting the spring bonnie suit on (The Killer)
Like how Michael Afton was scooped and posseded by Ennard,Could Have William suffered the same fate? He was scooped and possibly posseded by Bite Victim,this could be the reasen why olny kills kids.