The fact that Baasha becomes king after the conspiracy and the successful attack on Nadab and remains king for twenty-four years says something about his personality: a powerful leader, who spares nothing and no one.
The killing of Nadab was a takeover of the kingdom by Baasha, and this takeover followed the normal pattern for dynastic changes in the ancient Near East. The family members of the murdered king posed a threat to the new king. Therefore, the new king took measures to eliminate the threat by killing all who were related to the old king.
The massacre of the entire family of a vanquished and slain king is more common in those days. Such a total execution prevents revenge from the family of the murdered monarch. But of course, this act causes a lot of suffering and gives the new king a negative reputation as a cruel, ruthless ruler from the beginning. Such an act also lays in the heart of the new leader a seed of doubt, suspicion: if you yourself have killed your predecessor and his family, what will happen to you and your family in due course? Always watchful, never at peace.
This practice was unjust, and one that David did not follow in the case of the family of Saul. Despite the injustice of it, it served as a fulfillment of the prophecy concerning the judgment that was to fall upon the descendants of Jeroboam.
On the prophet Ahijah and his ominous prophecy, see 1 Kings 14:1–16. Strictly speaking, 1 Kings 15:29–30 does not state that Baasha was commanded by God to kill the entire family of Jeroboam.
28 So Baasha killed him in the third year of Asa king of Judah and reigned in his place.