Charles Ellicott Commentary 1 Kings 15:29

"And it came to pass that, as soon as he was king, he smote all the house of Jeroboam: he left not to Jeroboam any that breathed, until he had destroyed him; according unto the saying of Jehovah, which he spake by his servant Ahijah the Shilonite;" — 1 Kings 15:29 (ASV)

According to the saying of the Lord. —See 1 Kings 14:10–14. There seems no reason to suppose that Baasha had any formal mission of vengeance, or that his conspiracy and assassination were due to any motive but his own ambition. The contrary, indeed, may be inferred from the declaration of 1 Kings 16:7, that the judgment on Baasha was in part because he killed Nadab and his house. Sin which works out God’s purpose is no less truly sin. Of Baasha we know nothing, except his attempt on the independence of Judah, and its failure (1 Kings 15:16–22).