Shule’s sons had earlier assassinated Noah (Ether 7:18), showing that capturing the king was not always the first option. In that case, Noah was killed as part of Shule’s rescue, restoring Shule to his throne but also somewhat puzzlingly leaving Noah’s kingdom and power base intact.
Here, Esrom and Coriantumr choose not to exercise the option of killing Jared. They had mounted a successful coup. Keeping Jared alive but obviously subservient to them must have had some political value. As it turns out, it also had its risks.