to die
殂 centers on the act of dying or passing away. It is a single, direct meaning with no extended abstract senses.
殂 combines 歹 (death) with 且, which likely contributes the sound. The exact historical development of the compound is uncertain.
The left side 歹 is associated with death, and the right side 且 looks like a stack of bones or a coffin. Together they paint a clear picture of dying.
For ソ, imagine a soul softly leaving the body: the whispered 'so' of the departing soul cues ソ, while the death radical 歹 marks the end.