Or HaChaim on Exodus 21:12/אור החיים על שמות כ״א:י״ב

1 מכה איש ומת, He who strikes a man and he dies as result, etc. This means that death need not occur immediately, such as when one slaughters or chokes someone to death. The striker is culpable even if death is delayed but occurs directly as a result of his blow. The only exception to this rule is if the stricken person had recovered sufficiently to be able to stand on his feet and leave his home unassisted (verse 19). The new element in this legislation is that if the striker had struck the victim unintentionally he is not obliged to flee to a city of refuge unless death occurred as an immediate result of the blow (compare Gittin 70). The Talmud states there specifically that if the victim had vital parts such as most of the windpipe and the gullet severed the killer is not consigned to the city of refuge as we consider the possibility that his victim did not die immediately, or contributed to his accelerated death by making a wrong move. Tossaphot comment that this rule applies only in the case of an involuntary killing. They derive this from Numbers 35,23: ויפל עליו וימות, "he cast it upon him whereupon he died." This means that death did not follow immediately. Although a person who has done this intentionally will be considered guilty of murder even if the victim did not die immediately, we distinguish between the laws of confinement to a city of refuge and the laws dealing with murder. This corresponds precisely to what we wrote that our verse deals with someone who attacked someone else intentionally. If the Torah wanted to include unintentional killing it would have had to write הורג איש instead of מכה איש. As it is, even if the attacker set out to kill now but death occurred only several days later, the death penalty applies. Concerning unintentional killing, the Torah speaks of והאלוקים אנה לידו, "and G'd caused it to happen by his hand;" this means that death occurred by means of the hand of the killer himself, not a delayed effect. You will find this confirmed by Maimonides' ruling in chapter five of his Hilchot Rotzeach. Rabbi Shlomo Aderet, who frequently disagrees with Maimonides, disagrees in this instance also. I find his reasoning quite unacceptable. This is not the place to evaluate the finer points of their dispute, however.

2מות יומת, he shall surely be put to death. The reason the Torah repeats this word as well as in such instances as someone either cursing or killing his father or mother, and someone guilty of kidnapping, needs to be analysed. Remember that it is the prerogative of G'd, the judge of the whole universe, to kill perpetrators of evil. However, instead of doing so personally, G'd has assigned judges of flesh and blood to judge part of the crimes for which people deserve to be killed. These judges are to hand down death sentences for crimes/sins listed in the Torah as carrying the death penalty. In some instances G'd has revealed why certain crimes carry the death penalty, i.e. one to be administered here on earth, while in other instances He has reserved the right to execute the guilty party Himself in His own good time. In all instances where the Torah speaks about the penalty being כרת, such as a husband who has marital relations with his wife while the latter is menstruating, G'd himself is the executioner. Ketuvot 30 informs us that the fact that nowadays we do not have a court authorised to impose and carry out the death penalty, does not mean that the party guilty of a sin for which the Torah legislates the death penalty will go unpunished. In the case of an ox which has gored a human being (the owner having been negligent), the Torah states that the owner should be executed in addition to his ox although such a law is not enforced by a human tribunal (Mechilta) but only by Heaven. Our sages in Baba Metzia 31 elaborated further on this subject.