The kids epically trolled him on Xbox live one too many times. This made him so mad he decided he'd troll them back by precisely planning out how he was gonna murder them at their favorite pizzeria.
@SuperSajanBonnie But Charlotte Emily died first then Fredbear killed Crying Child/Evan/ then William murdered 5 children and after 5 missining children Elizabeth is killed by Circus Baby.
I’m pretty sure it was for revenge or jealousy in Charlie’s case but after that I pretty sure Willam just liked it (or maybe he was just insane who knows nothings confirmed)