Albert Barnes Commentary Jeremiah 40:14

"and said unto him, Dost thou know that Baalis the king of the children of Ammon hath sent Ishmael the son of Nethaniah to take thy life? But Gedaliah the son of Ahikam believed them not." — Jeremiah 40:14 (ASV)

It is difficult to say what motive Baalis could have had for murdering Gedaliah. As an ally of Zedekiah (Jeremiah 27:3), he may have had a spite against the family of Ahikam for opposing, as they most probably did at Jeremiah’s urging, the league proposed (Jeremiah 27:1). Ishmael’s motive was envy and spite at seeing a subject who had always opposed the war now invested with kingly power, in place of the royal family.