Malbim: It says in Divrei ha'Yamim II, 21:4, 13, 16-19 that he killed all his brothers, Bnei ha'Melech, and he enticed the residents of Yehudah [to do idolatry]. Hashem aroused against him the Ru'ach of the Pelishtim and Aravim, and they conquered Yehudah and captured all his wives and sons. Only his youngest son Yeho'achaz ¹ remained, and Yehoram died amidst bad illnesses.