This queston is discussed by the Mishneh LeMelech, Hilchos Rotzeach 1:15. He writes that it is logical to say that if the person who killed b'Shogeg then killed the Goel Hadam to prevent himself being killed, he would not be liable to the capital penalty for defending himself. Mishmneh Lemelech writes that he cannot adduce a conclusive proof for this Halacha but there is support for it from the Gemara Sanhedrin 82a which states that if Zimri would have turned round and killed Pinchas, Zimri would not have been put to death for doing so. Mishneh LeMelech concludes that there is only permssion for the Goel Hadam to kill the murderer b'Shogeg but it is not a Mitzvah to do so, and therefore the murderer b'Shogeg is allowed to defend himself.

Good Shabbos

Dovid Bloom

According to the Ponim Yofos everyone agrees that if the inadvertent murderer kills the Goel Hadam, in self-defence on his way to the refuge city, he is exempt:-

1) The Ponim Yofos; by the author of Haflo'oh; writes in Dvorim 19:6 that both opinions in Makos 10b concerning whether the latter verse refers to the Goel Hadam or the b'shogeg killer, maintain that if the Rotzeach kills the Goel Hadam in order to save his own life, that he is exempt.

2) The annotator on the Ponim Yofos points out that even though the Mishneh LeMelech; that we saw above; is in doubt concerning this question, this is no contradiction to the Ponim Yofos who writes that everyone agrees that he is exempt, because the Mishneh LeMelech was only discussing a Rotzeach who fled to the refuge city, and then left it without permission. Since he was not allowed to leave, it is possible that he has no right to defend himself by taking the life of the Goel Hadam. However, Mishneh LeMelech will agree that it is obvious, without any need of a proof, that if the inadvertent killer had not yet reached the Ir Miklat, the Goel Hadam may not kill him so the Rotzeach b'Shogeg is certainly allowed to save his own life by killing the Goel Hadam (see Rambam Hilchos Rotzeach 5:10).

Dovid Bloom