“may it fall.” More literally, “may it swirl around,” likely with the idea of a continuation, not a one-time action. And the “it” refers to the blood.
“and on all his father’s house.” David’s saying the bloodguilt for the death of Abner should alight on all Joab’s “father’s house” implicates Joab’s brother Abishai as well, and we learn from 2 Samuel 3:30 that Abishai was part of the plot to kill Abner. The Bible does not say what part Abishai played in Abner’s death; perhaps he was part of the delegation that brought Abner back to Hebron. David asks that the blood of Abner alight on all “his father’s house,” and he says that knowing that Joab’s mother is his sister, Zeruiah (2 Sam. 2:18; 1 Chron. 2:15-16), and therefore Joab and Abishai are his nephews.
“Let there not fail from the house of Joab.” This curse pronounced by David is very serious and is multigenerational. No doubt Joab committed murder, but does that warrant a curse upon his descendants forever such that they are sick, diseased, and hungry? This seems to be one of the places where David’s emotions overpowered his good judgment.
David pronounced a curse, but he did not move to execute Joab as a murderer, perhaps because of the ancient law of the avenger of blood, and that it could be argued that Joab acted as the avenger of blood for his brother Asahel. But it seems clear that Abner did not feel he needed protection from an avenger of blood because if he had then he would have gone to live in one of the cities of refuge in Israel, and apparently David did not feel that way either because if he had, he would not have had Abner come to Hebron to meet him.
“who holds a spindle.” The scholars debate whether this should be “crutch” or “spindle,” and the versions and commentaries are divided. Those who argue for “crutch” point out that it fits well in the context and there is lexical grounds for the translation (cp. CJB, JPS, NIV, NLT). Those who argue for “spindle” also say that it fits well in the context—a curse that Joab’s male descendants would not be warriors but would do women’s work—and also claim lexical support for their position (cp. HCSB, ESV, NET, NRSV). The decision is difficult, and what David meant is still uncertain. The REV went with “spindle.”