Gemara

Chachamim learn from "v'Chi Yinatzu Anashim Yachdav" that if Levi intended to kill Reuven and killed Shimon, he is liable.

(R. Elazar): The verse discusses a fight to kill - "v'Im Ason Yihyeh v'Nosata Nefesh Tachas Nafesh" (Beis Din kills only one who intended to kill).

R. Shimon says that it refers to paying money, like Rebbi.

(Beraisa - Rebbi): "V'Nosata Nefesh Tachas Nafesh" refers to money. It says 'Nesinah' here, and also about Demei Vlados (payment for causing a miscarriage). Just like there it refers to money, also here.

(Rava): Tana d'Vei Chizkiyah argues with Rebbi and with Chachamim.

(Tana d'Vei Chizkiyah): "Makeh Adam... u'Makeh Vehemah" equates striking (killing) people and animals. One who strikes an animal always pays, whether he was Shogeg or Mezid, with or without intention. Similarly, one who strikes a man never pays, without distinction.

This cannot mean 'without intention at all', for that is Shogeg! Rather, he intended for Reuven and killed Shimon. If Beis Din kills him, we would not need to teach that he does not pay! Rather, it teaches that he is not killed, and he does not pay money.

Bava Kama 42a (Abaye and Rava): When men fight, if the woman lives, they pay (for the fetus). If she dies, they are exempt.

(Rav Ada bar Ahavah): The exemption of men does not depend on a fatality, rather on intent! Rather, when men fight and intend for each other, even if she dies, they pay. When they intend for her, they are exempt (if she dies).

A Beraisa supports Rav Ada.

44b (Mishnah): If an ox intended to kill an animal, and killed a Yisrael... it is exempt.

(Inference): Had it intended to kill Reuven and killed Shimon, it would be liable! Our Mishnah is unlike R. Shimon (who exempts in such a case).

R. Shimon expounds "the ox will be stoned, and also its owner will die." An ox is killed like its owner, i.e. only if it killed its intended victim.

Kesuvos 35a - Question: Does anyone say that one who transgressed a Chiyuv Misah b'Shogeg must pay?! Tana d'Vei Chizkiyah taught... one who strikes a man never pays, without distinction!

Rishonim

Rambam (Hilchos Na'arah Me'orasah 1:13): One who strikes a man is exempt from paying whether he was Shogeg or Mezid.

Rambam (Hilchos Rotze'ach 4:1): If Levi intended to kill someone and killed someone else, he is exempt from Misas Beis Din, payment and exile (in a refuge city, if he was Shogeg). Therefore, if one threw a rock into a crowd of Yisre'elim, he is exempt from Misas Beis Din.

Rebuttal (Ra'avad): This is like R. Shimon. The Halachah does not follow him.

Kesef Mishneh: Rava (in Sanhedrin) holds like Tana d'Vei Chizkiyah. The Halachah follows follows Rava, for he is Basra (the chronologically last opinion). Also, Kesuvos 35a considers Tana d'Vei Chizkiyah to be the primary opinion, like Tosfos (35a DH u'Mi) says (because he learned the authenticated Beraisos in the academy of his father R. Chiya). This answers the Ra'avad's question. However, the end of the Rambam is difficult, for we exempt one who threw a rock into a crowd only according to the one who exempts for killing someone other than he intended for! The Rambam should have taught a bigger Chidush, that he is exempt even if he intended for one of two people, or he killed Shimon, thinking that he is Reuven! Rather, the Rambam exempts one who threw into a crowd because he cannot be warned, unlike Rashi (Kesuvos 15a DH Amrei). Even though the Rambam says 'he intended to kill someone and killed someone else', like R. Shimon said, he does not rule like R. Shimon's reason.

Rebuttal (Lechem Mishneh Hilchos Chovel u'Mazik 1:14): The Rambam rules like the Stam Mishnah in Bava Kama. He does not equate animals to people like R. Shimon, but he rules like R. Shimon regarding people. If Chachamim exempt due to Safek warning, l'Halachah they agree with R. Shimon!

Rambam (Hilchos Chovel u'Mazik 4:5): If one struck a pregnant woman and made her miscarry and she died, even if he was Shogeg he does not pay anything. "If there will be no Ason, he will pay" - whenever there is death, we do not distinguish between Shogeg and Mezid to obligate paying.

Rambam (6): This is if he intended to hit her. If he intended to hit someone else, even if she died he pays for the fetus. Because he did not intend for her, this is like a matter without Misas Beis Din.

Rebuttal (Ra'avad): This is like R. Shimon, who exempts one who killed a different person than he intended for. The Rambam himself rules like Chizkiyah, who exempts from payment regardless of intent!

Defense (Magid Mishneh): The Rambam rules like Chizkiyah, who exempts Chayavei Misos Shogegim from payment, and he rules like Rav Ada, that when men fight, intending for each other, even if she dies, they pay. This is like Rebbi. This part is unlike Tana d'Vei Chizkiyah. These two parts do not depend on each other. Even though Rav Ada holds like R. Shimon, who exempts one who killed someone he did not intend for (and normally the Halachah follows Chachamim), the Rambam rules like R. Shimon (Hilchos Rotze'ach 4:1). It seems that he learns from Bava Kama 42a, in which Rav Ada challenges Abaye and Rava, who hold like Chachamim. We conclude like Rav Ada. There are other opinions.

Lechem Mishneh (Hilchos Chovel u'Mazik 4:5): The Rambam does not rule like Rebbi! Rebbi obligates money, and the Rambam (Hilchos Rotze'ach 4:1) exempts!

Tosfos (Kesuvos 35a DH u'Mi): Why did the Gemara ask 'does anyone obligate Chayavei Misos Shogegim to pay?! Rebbi and Chachamim argue with Tana d'Vei Chizkiyah! Rather, all agree that one does not pay the value of the one he intended to kill. When he killed someone else, Tana d'Vei Chizkiyah totally exempts, Chachamim obligate Misah, and Rebbi obligates other money (Demei Vlados).

Gra (10, Likut 2): The Rambam rules like Tana d'Vei Chizkiyah, but not regarding Demei Vlados, rather, like Rav Ada.

Gra (10, Likut 3): The Rambam rules like Rav Ada. He holds that Tana d'Vei Chizkiyah does not disagree. 'Whether or not he intended' refers to exemption for paying for the death of the woman herself, like Sanhedrin 79b connotes. The Ra'avad holds like Tosfos.

SMA (CM 423:15): Even though the Rambam exempts one who killed someone he did not intend for, he says that this is like a matter without Misas Beis Din, for also there is no Misas Beis Din for Shogeg, but then we exempt from Demei Vlados because he would have been killed had he been Mezid. One might have thought that the same applies when he did not intend for her. The Rambam teaches that this is not so. In this case, it is considered that there is no Misas Beis Din in this matter at all.

Poskim

Shulchan Aruch (CM 423:4): If one struck a pregnant woman and made her miscarry and she died, even if he was Shogeg he does not pay anything. "If there will be no Ason, he will pay." This is if he intended to hit her.

SMA (14): Even though the verse discusses men trying to kill each other and one killed her without intent, the Diyuk is that if someone died, they are exempt from Demei Vlados due to Kam Lei bid'Rabah Minei. This does [not] apply when they did not intend for her, for he holds that there is never Misah in such a case. When he intended for (to hit) her, even if he was Shogeg about her death, the verse teaches that whenever he would be killed if he were Mezid, even if he was Shogeg, he is exempt from paying.

Shulchan Aruch (ibid): If he intended to hit someone else, even if she died he pays for the fetus. Because he did not intend for her, this is like a matter without Misas Beis Din.

Rema: Some exempt even if he did not intend to hit her.

Beis Yosef (EH 177:4 DH v'Im Ba (2)): Rashi says that Chayavei Misos Shogegin is when he was not warned. The Rambam exempts from payment whether or not he was warned, and whether he was Shogeg or Mezid.