“he struck down all the house of Baasha.” In this context, “the house of Baasha” is the dynasty of Baasha. Zimri did not kill Baasha, he had died earlier. Zimri killed Elah, Baasha’s son, but Zimri did not want any rivals to arise from Baasha’s descendants, so he killed them all and thus destroyed the house of Baasha. The dynasty of Baasha was the second dynasty to arise in Israel. The first was the dynasty of Jeroboam I, the second was the dynasty of Baasha, and the third was the dynasty of Omri, which is sometimes referred to as the dynasty of Ahab, Omri’s son, due to Ahab’s prominence (see commentary on 2 Kings 9:9).
“who pisses against a wall.” A crass idiom and cultural way of referring to the men. 1 Kings 16:11 seems to indicate that Zimri killed all the men related to or close to Baasha, but left the women alive.
“kinsman-redeemers.” A kinsman-redeemer could avenge Baasha’s death, so Zimri had them all killed.