The execution of Joab is more than a mere carrying out of justice; it is the settling of a deep moral and spiritual imbalance caused by years of bloodshed. By putting the former army commander to death, the royal house is cleansed of the guilt of murder, transferring the absolute responsibility onto Joab and his future generations. However, a question arises regarding the necessity of punishing both Joab and his descendants, given that Joab pays for his crimes with his own life.
The primary approach among commentators [מצודת דוד, אברבנאל] is that Joab’s execution does not serve as a complete atonement for his past murders of Abner and Amasa. Because his death sentence is heavily influenced by his recent involvement in a rebellion, it does not fully pay the moral debt of the innocent blood he previously shed. Therefore, the guilt of those murders continues to haunt Joab even after his death and permanently attaches to his descendants [מצודת דוד]. This lasting punishment serves as a direct continuation of the harsh curse David originally placed upon Joab's family [רד״ק, אברבנאל].
David's curse condemned Joab's lineage to suffer from five specific afflictions: bodily discharges, leprosy, leaning on a spindle, falling by the sword, and starvation. The Sages explain that these afflictions specifically target and strip away the five natural blessings a parent hopes to pass down to their children: strength, beauty, wisdom, long life, and wealth. A bodily discharge drains physical strength, while leprosy destroys physical beauty. The imagery of holding a spindle represents the loss of wisdom, depicting a foolish person leaning on a fragile, dry piece of wood. Alternatively, these curses act as a precise reflection of Joab's crimes [אברבנאל]. Because Joab proudly let his victim's blood drip onto his shoes, his descendants are cursed with dripping bodily discharges. Because he isolated his rival out of jealousy, his descendants will be isolated as lepers outside the camp. Because he arrogantly trusted in his sword, his lineage is left holding a simple wooden spindle instead of a weapon. Furthermore, since he cunningly murdered to avenge a brother who fell in battle, his descendants will also fall by the sword. Finally, because Joab killed out of fear that a time of peace would end his lucrative spoils of war, his descendants are condemned to starve.
Following the condemnation of Joab, a lasting blessing of peace is bestowed upon the royal family. This blessing serves to publicly clear David of any lingering guilt for failing to execute Joab during his own lifetime [אברבנאל]. Extending to the entire royal dynasty, the blessing ensures an era of absolute peace that secures both physical well-being and total political stability. With the evil finally purged, there are no remaining obstacles to harmony [מצודת דוד].
Yet, beneath this promise of peace lies a deeply complex reality. Ultimately, King Solomon’s own descendants end up absorbing these very curses [אברבנאל]. This tragic reversal occurs because Solomon exercises his absolute royal power to execute Joab right next to God's altar, severely violating the sanctity of the Temple. Furthermore, the execution is partially based on Joab's role in a rebellion, an offense that does not strictly warrant the death penalty from a purely legal standpoint. Therefore, in a strict measure of divine justice, Solomon's lineage is destined to suffer the exact fate he imposed on Joab's family. They would endure impurities that distance them from the Temple, lose their royal scepter in exchange for a wooden spindle, fall by the sword, and suffer from hunger.