He did what was evil in the sight of the LORD is the same evaluation that was given to Jehoram in 2 Chr 21.6 (see the comments there).

As the house of Ahab had done: See the comments on 2 Chr 21.6.

For after the death of his father they were his counselors, to his undoing: His father refers to Jehoram. The pronoun they refers to members of the family of Ahab. Good News Translation makes this clear by saying “other members of King Ahab’s family.” To his undoing is more literally “to his destruction” (King James Version, NASB). New Century Version translates this whole sentence as “They [Ahab’s family] gave advice to Ahaziah after his father died, and their bad advice led to his death.”

Quoted with permission from Omanson, Roger L. and Ellington, John E. A Handbook on 1-2 Chronicles, Volume 1. (UBS Helps for Translators). Miami: UBS, 2014. For this and other handbooks for translators see here .