From BiblePortal Wikipedia

Fausset's Bible Dictionary [1]

Son of Shimeath the Ammonitess; one of Joash's murderers ( 2 Kings 12:21). Zabad in  2 Chronicles 24:26 is a transcriber's error for Zachar contracted for Jozachar. As Joash had forsaken Jehovah for foreigners' idols, so his doom was inflicted by two sons of foreign women.

Smith's Bible Dictionary [2]

Joz'achar. (Whom Jehovah Has Remembered). One of the murderers of Joash, king of Judah.  2 Kings 12:21. The writer of the Chronicles,  2 Chronicles 24:26, calls him Zabad . (B.C. 837).

Morrish Bible Dictionary [3]

One of those who killed Joash, or Jehoash, king of Judah.  2 Kings 12:21 . He is called ZABAD in  2 Chronicles 24:26 .

Holman Bible Dictionary [4]

 2 Kings 12:21JehozabadJozabad

Easton's Bible Dictionary [5]

 2 Kings 12:21 2 Chronicles 24:26

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [6]

(Heb. Yozakar', יוֹזָכָר , Jehovah - Remembered; Sept. Ι᾿Ωσαβάδ v.r. Ι᾿Ωζαχάρ ), the son of Shimeath, an Ammonitess, one of the two servants who assassinated Jehoash, king of Judah, in Millo ( 2 Kings 12:21). In the parallel passage ( 2 Chronicles 24:26) the name is erroneously written Zabad. BC 837. "It is uncertain whether their conspiracy was prompted by a personal feeling of revenge for the death of Zechariah, as Josephus intimates ( Ant. 9, 8, 4), or whether they were urged to it by the family of Jehoiada. The care. of the chronicler to show that they were of foreign descent seems almost intended to disarm a suspicion that the king's assassination was an act of priestly vengeance. But it is more likely that the conspiracy had a different origin altogether, and that the king's murder was regarded by the chronicler as an instance of divine retribution. On the accession of Amaziah the conspirators were executed."