John Gill 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)

And it came to pass, when he reigned, that he smote all the
house of Jeroboam
That he might have no rival, or any that could pretend any title to the crown:

he left not Jeroboam any that breathed, until he had destroyed him,
according to the saying of the Lord, which he spoke by his servant
Ahijah the Shilonite ;
not that his intention in destroying Jeroboam's family was to fulfil that prophecy, but so it was eventually; see (1 Kings 14:10 1 Kings 14:14) .