Notes on the Bible, by Albert Barnes, [1834], at sacred-texts.com
For "42" read "22" (see the marginal reference). Ahaziah's father, Jehoram, was but 40 when be died Ch2 21:20.
The destruction of Ahaziah was of God - i. e. his untimely end was a judgment upon him for his idolatry.
Compare the marginal reference. Ahaziah after remaining a while at Megiddo, removed to Samaria, where his wounds could be better cared for and concealment might be easier; Jehu's emissaries discovered him there; they took him to Jehu, who happened at the time to be at Megiddo; and then and there Jehu put him to death. The narrative here is therefore supplementary to that of 2 Kings, and finds its proper place between the clause," He fled to Megiddo," and the words "and died there."
And when they had slain him, they buried him - Jehu's emissaries killed him but allowed his servants to bury him (see Kg2 9:28).
No power ... - As Ahaziah was but twenty-three at his death (Ch2 22:2 note), he had no grown-up son to take the crown.
Compare the marginal reference and notes.