וַיֹּאמֶר֩ יְהֹוָ֨ה אֱלֹהִ֥ים ׀ אֶֽל־הַנָּחָשׁ֮ כִּ֣י עָשִׂ֣יתָ זֹּאת֒ אָר֤וּר אַתָּה֙ מִכָּל־הַבְּהֵמָ֔ה וּמִכֹּ֖ל חַיַּ֣ת הַשָּׂדֶ֑ה עַל־גְּחֹנְךָ֣ תֵלֵ֔ךְ וְעָפָ֥ר תֹּאכַ֖ל כָּל־יְמֵ֥י חַיֶּֽיךָ׃
האָט גאָט דער האַר געזאָגט צו דער שלאַנג: װײַל דו האָסט דאָס געטאָן, זאָלסטו זײַן פֿאַרשאָלטן פֿון צװישן אַלע בהמות, און פֿון צװישן אַלע חיות פֿון פֿעלד; אױף דײַן בױך זאָלסטו גײן, און שטױב זאָלסטו עסן אַלע טעג פֿון דײַן לעבן.
Rashi
כי עשית זאת BECAUSE THOU HAST DONE THIS — From here we infer that we should not occupy ourselves with what may be in favour of one who seduces people to idolatry, for had He asked it, “Why hast thou done this?”, it could have answered Him, “When the words of the teacher and those of the pupil are contradictory, whose orders should be obeyed?” (Sanhedrin 29a). (i. e. if You told them one thing and I another, should they not have obeyed You?).
Ramban
FROM AMONG ALL CATTLE, AND FROM AMONG ALL BEASTS OF THE FIELD. From a study of this verse, Rabbi Yehoshua the son of Chananya derived the fact that a serpent gives birth once in seven years;394Bechoroth 8a. See my Hebrew commentary, p. 41. this they investigated and found to be so.395See Bereshith Rabbah 20:7. For the Midrashic interpretations of Scripture and their allusions are all traditional, and they found in them profound secrets on procreation and all matters, as I mentioned in my introduction.
Radak
ויאמר כי עשית זאת, he did not have an answer to this, no excuse whatsoever. Adam and Chavah, though their excuse was indeed feeble, at least had some sort of an excuse, whereas the serpent had no excuse whatsoever. This is why G’d immediately proceeded to curse the serpent, without even waiting if the serpent would try and come up with an excuse. This is what Solomon had in mind when he said in Kohelet 10,11: ואין יתרון לבעל הלשון, “a slanderer (the serpent) has nothing going for him at all (having bitten without provocation).” (compare Taanit 8) Our sages (Sanhedrin 29) say that the serpent did have an answer ready but that G’d did not give it a chance to use it seeing it had initiated this seduction. (The Talmud, quoting one of the possible excuses of the serpent, cites the well known phrase: “if the words of the student are at variance with those of the teacher, whose words does one have to heed?” Whatever excuse the serpent could have used, it had no answer to the accusation why it had engaged in seduction.)