Albert Barnes Commentary 2 Chronicles 21:4

"Now when Jehoram was risen up over the kingdom of his father, and had strengthened himself, he slew all his brethren with the sword, and divers also of the princes of Israel." — 2 Chronicles 21:4 (ASV)

The execution of several “princes of Israel” (that is, of Judah; see the note on 2 Chronicles 20:34) implies that Jehoram’s brothers found supporters among the chief men of the country, and that Jehoram’s sole sovereignty was not established without a struggle.