Melakhim describes Yoash as a good king. A different story is described in great detail in Divrei Hayamim. After Yehoyada's death, Yoash abandons the guidance of the Kohanim, turns to idolatry and murders the prophet Zekharya in the courtyard of the Beit HaMikdash - the son of the man who saved him and made him king. Just as he betrays, his servants betray him and kill him after he is injured in a battle against Hazael - King of Aram who despite being outnumbered defeats Yoash and walks away with great spoils. Different theories are suggested as how to reconcile the two accounts but it remains unclear why this episode is hardly even alluded to in Melakhim.

Courtesy of the Virtual Beit Midrash, Yeshivat Har Etzion