This is gonna be a short and inconsequential one but wanted to put it out there
I believe that William is so cold and hollow behind his mask that he never understood human emotion. This is why he does everything he does in FNAF. He’s still that theater kid everyone jokes about but it’s just his method to study emotion
By day he entertains children to study joy and at night, tortures his victims, possibly even his own children, in nightmare experiments that study their fear. This gives a reason why everything surrounds a pizza restaurant (did that never confuse anyone else, like why a pizza restaurant owner is this almost comically evil killer scientist). He uses his business to conduct all his experiments later as he soon discovers spirits, remnant, and agony.
Agony is the catalyst for EVERYTHING that happens after FNAF 4. After killing Charlotte, he’d discover the power to haunt via the pure emotion brought on from a traumatic event. Agony is the diving board William takes from studying emotion to studying the supernatural. Becoming immortal was just a step on this journey, I never liked this endgame motive for William. He was always just a curious and sadistic character to me