“… In being like the house of Jeroboam, and also because he destroyed it.” This does not mean that Elah killed Jeroboam, but that Baasha, father of Elah, killed the son of Jeroboam. And he is not threatened with evils by the prophet because he killed him but because he does not fear the punishment which was performed by his hands as a consequence of Nadab’s sins. And since he is at the moment imitating the evil actions of that one, he will undergo the same punishment as well.
Source: BOOKS OF SESSIONS 1 KINGS 16:7