Lévitique 20:9

For the young man or the old man who curseth his father or his mother by the revealed Name, shall be verily put to death by the casting of stones; because he hath cursed his father or his mother, he is guilty of death.

Scripture now mentions the punishments for the crimes that were listed at the beginning of the parasha , starting with “Each of you will be in awe of his mother and father” [19:3]. Here the father is mentioned first, out of respect.Any man who curses his father and his mother Scripture employs ellipsis, and means “Any man who curses his father, and any man who curses his mother”.

he cursed his father or his mother That is to say: He has done a horrible thing.

he alone is to blame for his death [literally: his blood is with him] compare, “his blood is on his head” [Joshua 2:19]. The tradition which we have received from our ancestors, and upon which we rely, sees in this phrase an allusion to the form of execution which does not cause external bleeding — i.e., stoning. Without the tradition, we would not know how to go about executing someone. We would also not know how old a person must be to be responsible for his crimes, since Scripture does not say so explicitly.

FOR WHATSOEVER MAN THERE BE THAT CURSETH HIS FATHER OR HIS MOTHER. The meaning thereof is that it refers back to the beginning of the section where He stated, Ye shall fear every man his mother, and his father, and He now states here, for whatsoever man there be that does not obey [this commandment] and curseth his father, or his mother, he shall surely be put to death, and by way of the Truth, [the mystic teachings of the Cabala], this verse is here stated because He had said [above], Sanctify yourselves therefore, and by ye holy; for I am the Eternal your G-d, and I am the Eternal Who sanctify you, meaning that it is the Glorious Name Who sanctifies us, for He is our Father, and our Redeemer from everlasting, and it is His Name; therefore he who curses those who participated in his formation, is liable to death. This is the reason for the law [of the punishment] of the adulterer and adulteress, [in the following verse], which He placed near here before [the law of] all forbidden relations [mentioned further on in this section]. I have already alluded to this above.
Now Scripture mentions here some of the forbidden relations in order to make [one who has intercourse with them] liable to death, these being a man’s wife, a father’s wife, a daughter-in-law, a male, a woman and her mother, and the same law applies to a woman and her daughter and her daughter’s daughter, for He mentioned here only some of the relations of a wife, but the same punishment applies to all those mentioned there in the admonition. And surely this applies all the more so to one’s own relatives, such as his son’s daughter and his daughter’s daughter mentioned there,, Verse 10. and needless to say, his [own] daughter. On all these matters there are also Rabbinical interpretations to establish their punishment. Similarly He mentioned here the punishment of death for lying with a beast. He mentioned excision again in the case of a menstruant [although this has already been included in a general statement above, 18:29], in order to declare him liable for mere sexual contact without completion of the act, this being the sense of the expression, he hath bared her fountain. Similarly [He has stated] in the case of a mother’s sister and a father’s sister, for he hath bared his near kin. He mentioned an uncle’s wife and also a brother’s wife, in order to declare them liable to [the punishment of] dying childless, …

דמיו בו, his blood is on his own head. He made himself guilty of the death penalty.

כי איש איש אשר יקלל, proof that I, G’d, am so insistent that your sanctity be manifest by your seed being genealogically pure is that the penalty for non observance is the execution of the person who curses his parents. The normal scenario which leads to a son or daughter cursing their parents has to do with legitimacy or otherwise of their offspring. When a son or daughter are the product of unions forbidden under the laws of incest, children of such unions have little reason to practice the commandment to honour their parents as it was from their parents that they learned to ignore Torah legislation. Solomon’s well known instruction שמע בני מוסר אביך ואל תטוש תורת אמך, “heed the moral instruction of your father, and do not ignore the teachings of your mother” (Proverbs 1,8) are most difficult to honour when they know they have seen the light of day only because their parents chose to ignore this very instruction.