John Gill Commentary 2 Kings 14

In the second year of Joash son of Jehoahaz king of Israel
reigned Amaziah the son of Joash king of Judah .
] As Joash king of Israel began to reign in the thirty seventh of Joash king of Judah, (2 Kings 13:10) , reigned forty years, Amaziah must therefore begin his reign in the fourth of Joash king of Israel; this therefore must be understood of his second year after he reigned alone, for he reigned two or three years in his father's lifetime.

He was twenty-five years old when he began to reign, and reigned twenty-nine years in Jerusalem. Fourteen years contemporary with Joash king of Israel, who reigned sixteen years (2 Kings 13:10), and fifteen after him (2 Kings 14:17). And his mother's name was Jehoaddan of Jerusalem, of whom we nowhere else read.

And he did that which was right in the sight of the Lord
. At least externally, and at the beginning of his reign:

yet not like David his father ;
not with a perfect heart, with that sincerity and uprightness as he did, see (2 Chronicles 25:2) ,

he did according to all things as Joash his father did ;
who at first reigned well, and then fell into idolatry, as this his son did.

Howbeit the high places were not taken away
Though he first did that which was right before God; nor did his father take them away; see (2 Kings 12:3) .

And it came to pass, as soon as the kingdom was strengthened in
his hand
That he was well settled on the throne, and had a share in the affection of the people, and the idolatry and murder his father had committed were worn off of the minds of the people, and the friends of the conspirators against him were become few or none:

that he slew his servants that had slain the king his father ;
charged them with the murder in a court of judicature, obtained a sentence against them, and had them executed.