Most translators believe “sons” should be singular “son,” as the Septuagint reads. But it is not without precedence that monarchs have taken vengeance on family members of their opponents. Joash's grandfather and grandmother, Jehoram and Athaliah, both purged their families of potential rivals (II Chronicles 21:4; 22:10). Perhaps their examples inspired Joash to kill some of Jehoiada's family to somehow avert Zechariah's plea for divine repayment. If such were the case, plural “sons” would be correct.
