Can ghosts and gods in the underworld kill humans at any time?

No, at most, ghosts are ordered to take the souls of the dead. Ghosts and gods have no right. They just monitor the good and evil of people's behavior and record it, or sometimes give people a little warning in various ways. In addition to karma, non-human (ghosts) can't interfere with people's lives at will, otherwise they will be punished-this is also cause and effect.

If you are interested, you can read such good articles as "Yin Law is Ruthless" and "Jade Calendar Treasure".