I kill ... I wound ... I will make mine arrows drunk with blood, and my sword shall devour flesh. Deuteronomy 32:39-42
After God killed 500,000 of Israelite soldiers, he killed their leader, king Jeroboam.
Neither did Jeroboam recover strength again in the days of Abijah: and the LORD struck him, and he died. 2 Chronicles 13:20
You see, God hated Jeroboam. To God, Jeroboam was the worst person who had ever lived.
Go, tell Jeroboam, Thus saith the LORD .... thou hast ... done evil above all that were before thee. 1 Kings 14:7-9
(Why did God hate Jeroboam so much? Because after he became king he made two golden calves, put them on altars at Bethel and Dan, and led the people in sacrifices to them. See 1 Kings 12:28)
God had already killed Jeroboam's son, and after God killed Jeroboam, he killed the rest of his family.
I'm not sure why. I guess it's all a part of God's family values. This killing is a bit out of place chronologically. But I've decided to order God's killings according to the order that they occur in the Bible. It's hard to be sure of the correct chronology of mythical events.