Halakhah for Sanhedrin 144:4

From the laws of the commandment is that which they, may their memory be blessed, said (Sanhedrin 72b and see Mishneh Torah, Murderer and the Preservation of Life 1:6-7) that even if the pursuer is small and the pursued is bigger than him in every regard, everyone is obligated to save [the pursued], and even [at the expense of] the life of the pursuer. And to what do these words apply - that we save with the life of the pursuer? When it is impossible for us to save [the pursued] with one of the limbs of [the pursuer]; but if it is possible to save him with one of the limbs and he saved him with his life - this is spilling of blood (murder). And so did our Rabbis, may their memory be blessed, say (Sanhedrin 49a) about the death of Avner when Yoav killed him: As it is written there (II Samuel 3:27), "and he died for shedding the blood of Asahel, his brother." And the tradition came about this that Yoav had a claim against Avner for the blood of Asahel, and judged him in a case of the Sanhedrin (High Court) - which means to say that he killed him for a claim for which it would have been fitting to [receive] death according to the Sanhedrin: He said to him, "Why did you kill Asahel?" Avner said [back] to him, "He was a pursuer." Yoav said to him, "You should have saved yourself with one of his limbs." Avner said to him, "I did not know how to aim at him [in that way]." Yoav said to him, "You aimed onto his fifth rib! And you did not know how to aim at him?" And about this is it stated, "he died for shedding the blood of Asahel, his brother."