Sanhedrin 49A

תהא לוטא, ולא תהא לאטא.

Be the one who is cursed and not the one who curses, as a curse eventually returns to the one who curses.

אתיוה ליואב, דייניה. אמר ליה: מאי טעמא קטלתיה לאבנר? אמר ליה: גואל הדם דעשאל הואי. עשאל רודף הוה. אמר ליה: היה לו להצילו באחד מאבריו.

The Talmud returns to discuss the incident of Joab: They brought Joab before Solomon, who judged him. Solomon said to Joab: What is the reason that you killed Abner? Joab said to him: I was the blood redeemer of the blood of Asahel; when Abner murdered my brother Asahel I killed him, in fulfillment of my duty as his blood redeemer. Solomon said to him: But Asahel was pursuing Abner with the intention of killing him, and therefore he had the status of a pursuer. Since Abner killed Asahel in an act of self-defense, you had no right to kill him as a redeemer of Asahel’s blood. Joab said to Solomon: Abner could have saved himself by wounding Asahel in one of his limbs. Having failed to do so, he was guilty of murder, and I was therefore entitled to kill him.

אמר ליה: לא יכיל ליה. אמר ליה: השתא בדופן חמישית כיון ליה, דכתיב ״ויכהו אבנר באחרי החנית אל החמש״, ואמר רבי יוחנן: בדופן חמישית, במקום שמרה וכבד תלויין בו. באחד מאבריו לא יכיל ליה?

Solomon said to him: Abner was not able to injure Asahel, because he was running and could not aim with precision. Joab said to Solomon: Now Abner was able to aim and hit him precisely in the fifth rib, as it is written: “And Abner struck him with the butt end of the spear in the ḥomesh” (II Samuel 2:23), and R' Yoḥanan says that this means that he hit him in the fifth rib, the place where the gallbladder and liver hang. If Abner could aim with precision at the fifth rib, could he not have successfully aimed at one of Asahel’s limbs?