For a moment there, he considered going there and helping them. But when he heard about the good performance they were doing, he decided to stay where he was.
Right now Sloth and others were facing hard times dealing with the strong monsters here. And yet William knew this was great, as like this his path towards the monster gates was wide open.
He killed monsters until he reached the first gate. There he stood like a mountain, stopping any monster he spotted. He released his two monstrous weapons freely, letting them kill as they liked without worrying about their safety.
The dark leaders noticed what he was doing, but they decided to not do anything to stop him. Even if he showed weird power and killed tons of monsters, he was just affecting a single gate, nothing else.
Yet they didn't get that William alone was much more dangerous than any of the scary masters led by Sloth. As he stood close to the giant monster gate, he got the chance to inspect it thoroughly.