Having already stated the law by someone who strikes an animal (in Pasuk 18), why does the Torah repeat it here?
Why is one Chayav Misah for striking a person even if he does not kill him (since the Torah does not add the word 'Nefesh')?
Rashi: He is Chayav Misah, because the Pasuk is referring to a case where a son strikes his father.
Targum Onkelos and Targum Yonasan: The Pasuk is speaking about where the man killed the victim.
QUESTIONS ON RASHI
Rashi writes that the Seifa discusses a son striking his father. This was already taught above (Shemos 21:15). Why does the Torah repeat it here?
Rashi: It is taught with striking an animal, to teach us that (a) just as the latter discusses when the animal is alive, so too, one is Chayav Misah for striking one's parents only in their lifetime, but not after they are dead, 1 and (b) that just like one is liable for Makeh Beheimah only if he inflicted a wound, so too, one is Chayav for striking one's parent only if one inflicts a wound.
Moshav Zekenim: Above, it discusses striking "Aviv v'Imo." One might have thought that he is liable only for striking both. 2 Here it says "Makeh Adam" to teach that one is liable even for one.
Rashi: In spite of the fact that one is Chayav for cursing them even after they have died. Presumably, this is also how Targum Onkelos and Targum Yonasan will explain the fact that the Torah mentions "Makeh Beheimah", since they explain both Pasuk 18 and the current Pasuk where the striker killed the animal (Refer to 24:21:1:1)..
R. Yonason (Sanhedrin 85b) holds that the prefix Vov always includes even one of them unless it says "Yachdav". And there, R. Yoshiyah learns from a Hekesh to cursing parents (about which "Aviv v'Imo Kilel" (20:9) is extra to obligate even one of them! Perhaps R. Yonason learned from here that the prefix Vov always includes even one. (PF)
Rashi writes that one is liable for striking one's parents only in their lifetime. Why is this unlike cursing, for which one is liable even after death?
Moshav Zekenim: Cursing parents is equated to cursing Hashem, which applies forever.