for the band of men ... had killed all his older sons

This is background information about the events of 2 Chronicles 21:16 and 2 Chronicles 21:17. Ahaziah was called "Jehoahaz" in that earlier account. It is possible he changed his name when he became king. (See: rc://en/ta/man/jit/writing-background)

all his older sons

"all Jehoram's older sons"