The fulfillment of a divine decree often unfolds through ordinary human actions. The complete destruction of Baasha's dynasty served as the realization of a prophecy given earlier [מנחת שי]. Zimri did not carry out this destruction because he wanted to obey a divine command. His motives were entirely political, very much like the ambitions that drove Baasha before him. However, through these purely political power struggles, the words of the prophet were fully realized [ביאור שטיינזלץ].
The ruin of the royal house happened exactly as God had declared. It served as a direct punishment for the sins committed by Baasha and by his son Elah, who continued the wicked practices of his father [אברבנאל].