You can't rule the underworld if you have a soul.
If true, the rebellion forming in the underworld needed to be dealt with swiftly and his soul found.
The Lake of Souls in her underworld was always still.
Lost souls, shut out of the underworld, was the greatest nightmare for Death.
She hated not being able to read his mind and see his thoughts, the way she had for the thousands of years they served the underworld together.