Jeremiah 40:14
And said unto him, Dost thou certainly know that Baalis the king of the Ammonites hath sent Ishmael the son of Nethaniah to slay thee? But Gedaliah the son of Ahikam believed them not. And said unto him, Dost thou certainly know that Baalis the king of the Ammonites hath sent Ishmael the son of Nethaniah to slay thee? But Gedaliah the son of Ahikam believed them not.
Jeremiah 40 belongs after the account of Zedekiah's reign and in chronological sequence following Jeremiah 39, recounting events after Jerusalem's fall. The verse itself belongs to the historical account of the period before Jerusalem's fall, involving events around Gedaliah. Baalis the king of the Ammonites sent Ishmael the son of Nethaniah to slay Gedaliah the son of Ahikam, but Gedaliah believed not those who warned him. That sending represents the Ammonite king dispersing the Jews who had gathered in Judah after Babylon had carried the nation away.
The word for soul in the verse means the person himself, so to smite the soul is to kill the man, here to kill Gedaliah. A soul is therefore mortal, because it can be killed.