Yusuf Zuck knew that these damned people of Heaven weren't going to let him leave easily. Heaven was an organization, a group of dreamers who were mad, and now that he could be said to have the technique to bring the dead back to life, they all wanted to keep him for the sake of their own immortality.
This was also the main reason why Yusuf Zuck suddenly stopped treating wealthy patrons.
If he could make those doomed to die immortal, the world would be thrown into chaos. He would become prey, and every power, every nation, every organization would be after him.
That's why he had disappeared abruptly, because he didn't want to get caught in that vortex. If he really got sucked in, it would become even harder to extricate himself later on.