Sanhedrin 118

2:
The first category describes how someone may kill another and yet not be executed (by decapitation) because the death penalty cannot be invoked because of the status of the victim. At first this will surprise us, and even cause us indignation, but let us remember two facts: the assailant is a human being just as the victim was, and the assailant is alive whereas the victim is dead, and no action of the court can change that fact. Our mishnah brings three examples of victims whose death does not incur the death of their killer (despite the malicious intentions of the killer). A human being may not be put to death for killing an animal. (Even the explicit requirement of the Torah that the owner of an animal that killed a human being because it was not properly restrained [Exodus 21:29] is "explained" by the sages as requiring that the owner only be fined.) Also, someone who kills a newborn that is obviously not viable (cannot be expected to survive) is not guilty of murder. The Hebrew, Nefel, which I have translated as "an unviable newborn" refers to an extremely premature newborn that cannot survive.

3:
This leaves us with the third example: the killing of a non-Jew. A review of the exegesis involved will clarify that there is no value judgment here. In the case of the "stubborn and rebellious son" we noted that the sages deliberately (and, they admit, illogically) excluded daughters from this category, thus excluding half the population from the possibility of being charged with being "stubborn and rebellious". Similarly here, the rabbinic exegesis is designed to remove almost the whole population of the world from the list of victims whose murder requires decapitation. (This does not mean that there are not xenophobic elements in the rabbinic tradition, it just means that it is not so in this case.) The Torah states:

5:
Our mishnah actually sets forth two conditions which must be answered in order that the killing of one Jew by another constitute a capital offence. As is the wont of the Mishnah instead of stating a general rule it leaves us to derive the general rule from the specific examples that it brings. We can perhaps state the general rule as requiring the assailant to both intend to kill the victim and also uses a force that would be lethal for the intended victim (but not necessarily for anyone else). Intention can only be established beyond all doubt (all doubt, not reasonable doubt) by the requirement of forewarning: that two witnesses must warn the assailant that he is about to commit a crime whose punishment is death and the assailant must respond that he is well aware of that fact! (We have noted this on many occasions, and I still ask your indulgence: what happens when it is reasonably certain that the assailant is guilty but the legal conditions were not met will be discussed in mishnah 5 of our present chapter.) Section two of our mishnah discusses four possibilities: "He meant to land on his victim's waist a non-lethal blow but landed a lethal blow on his heart". Here the assailant did not intend to kill, so he is not culpable. "He meant to land a lethal blow on his victim's heart but landed a non-lethal blow on his waist, but the victim [nevertheless] dies". Here the blow was not lethal (the victim died from shock or some other reason. In section 3 of our mishnah "he meant to land on a large person a blow that would not be lethal on a large person but it landed lethally on a small person". The force of the blow was not lethal for the intended victim, and there was no intention in the case of the actual victim. "He meant to land on a small person a lethal blow but it landed on a large person, who [nevertheless] dies". Here there was no intention to kill the victim. Section four of our mishnah explains situations which do meet the required criteria. "He meant to land a lethal blow on his [victim's] waist but it landed fatally on his heart – he is culpable" because there is both intention and a blow that was lethal for the intended victim.