After Jacob kills the Soulless Ghost King in the most efficient way, he wants to get Elia and leave the city.
But it was already too late, and the Shadowless King and Black King had already captured Elia. Furthermore, they were already outside the king's chamber. If Jacob had shown himself, he knew he had no choice but to fight, which would've rendered everything he had done to get rid of Soulless Ghost King useless while also putting Elia's life at risk.
That's why Jacob quickly decided to feed the Soulless Ghost King's body to the Gluttony Mask. Although he knew there was a considerable risk of him losing control over the Gluttony Mask since Soulless Ghost King was a peak legendary king and probably a step away from becoming a Quasi-Myth, he still chose to go with it.
