Or HaChaim on Genesis 3:15/אור החיים על בראשית ג׳:ט״ו

1 ואיבה אשית בינך ובין האשה. And l shall place hatred between you and the woman. G'd applied the principle of גואל הדם, a relative avenging a killing who is permitted to kill the slayer without trial. This is so because the serpent's deed brought death to all of Eve's descendants. The nature of the revenge matches what the serpent had done. Humans will strike at the head of the serpents, seeing that the original serpent had initiated man's downfall and the head is the beginning of life, it is the area which is smitten by the angel of death. Whenever it attacks man, the serpent will aim at the heel because man in the end trailed, i.e. followed the advice of the serpent when sinning. The word "heel" may also simply be an allusion to the serpent's having lost its legs, being forced to move on its belly.

2You may well ask how could the Torah in the same breath as allowing man to avenge himself on the serpent permit it to attack man's heel? What kind of penalty is it for the serpent to be allowed to continue harassing man? The answer is simply that the serpent could have retorted that no one forced Eve to listen to its advice. Our sages (Kidushin 42) have formulated this by asking rhetorically: "When the instructions of the teacher conflict with those of the disciple, whose instructions is one to follow?" If Eve had followed this simple piece of logic there would not have been any unfortunate developments as a result of the serpent's argument. The serpent therefore felt aggrieved that Eve's lack of logic had caused it permanent harm. There is an allusion as to how long this condition of enmity between man and the serpent will continue. As long as man, i.e. Israel, is ראש, deserving of the title "head" by performing G'd's commandments, it will prevail over the tempter called serpent and smash it. On the other hand, when Israel ignores G'd's commandments, the serpent will bite it because Israel has itself become like a heel instead of like a head.