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.

The true lore revealed

William is just insane because his favourite child died (elizabeth btw)

@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.

There is not a definite answer. The reason why he did it is up to debate.

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)

I personally believe that William did it out of spite and grief after losing his children (and probably disowning Michael, losing his wife, and other things). (And/or) So he could make others feel his rage and pain. The same could be about when he killed Charlie, but we're not too sure about that one.

After the Missing Children incident, it could be seen that William, by this time, could have discovered Remanety or however it's spelled. Maybe he did another murder because of it, so he could experiment with it.

Or it could just be the same reason as previously stated.

That's my theory, however.

Thx guys! Although I like Kennedy's theory the best. :)

Thanks