פרשת בראשית תש"ל - קין וצאצאיו

According to the opinion of Benno Jacob in his commentary on Genesis (in German), the story of Cain is the conclusion of Adam's sin; so his punishment is somewhat parallel to Adam's punishment. And the commentator ends his words there as follows:

The relationship of the sin of Cain to the sin of Adam is like the relationship of the first and second tablet of the two tablets of the covenant.

Ibn Kaspi:

And this is a general matter; as all of the names of Cain's progeny have a root and a meaning in Hebrew – like the names of the children of Jacob. And it is all to instruct; as the Scripture explained with the children of Jacob. However it was not like this with the children of Cain, as [this] is not fitting for several reasons.

Cassuto, "From Adam to Noah":
In the same way that in verse 4:2, after the words, "and she continued to bear his brother, Abel," it is stated, "and Abel was a shepherd of sheep"; so too here, after the words, "and she conceived and bore Enoch," there is the similar wording of, "and he built a city." Hence in the same way as the subject of the verb, "and he was," is certainly the son that was born, so too here – it is not Cain that the verse is speaking about, but his son, Enoch. (In verse 2, the name of the son is [only] emphasized because of its opposition to the name, Cain; whereas here, there was no reason for it.)

R. S. R. Hirsch:
It does not state, "and he built a city," but rather "and he became the builder of a city." A new ambition was born with Cain: the city. Cain was disconnected from the ground, banished from the field, abandoned by God and excommunicated from the company of man. What else remained for him? He himself remained for him – his spiritual powers and the talents hidden in his personality remained. The natural result was – that he would become the builder of a city. The city – as opposed to a village – is disconnected from the ground and detached from agriculture. Its inhabitants make their livelihood from the powers of man. Agricultural produce comes to the city. But there, it is processed according to the needs of man – craft and industry imprint it with the stamp of human intelligence. Power, spirit and talent – these are the "fields" of the city-dwellers. And these are what continue to be developed, as is recounted immediately afterwards. The village is focused on the development of the ground; the city – on the development of man. The forces slumbering in man are awoken by the city – it is as if he is aroused... And the verse (Psalms 72:16), "and let men blossom up in cities like grass in the field" comes to be. Its simple meaning is like its plain reading – man blossoms in the city like the grass of the field. The contrast between the city and the village is also revealed in later laws: The tribe of Levi did not take a portion in the ground – but it was granted the Levite cities. The killer is exiled from the holy ground – in the same way as Cain was exiled from all of the earth – and is exiled to a city of refuge. Likewise, the homes of walled cities are judged as if they were disconnected from the ground – they pass from one hand to the other in perpetuity, like movable goods. And therefore it is characteristic that the first city was established by Cain, who was disconnected from the ground and dependent only upon himself. With the birth of his first son, he saw that he needed to secure his future; and he had no other path except to become the builder of a city.

Biur (R. Shlomo Dubno):
S.v. Forger: It is likely that it is a "flattened" noun [as in] (I Samuel 15:16), "and we proscribed the rest" [even though the noun would normally be understood as, more (yoter); when "flattened," it becomes rest (yeter)]. [Hence it] means to say, he sharpened all the crafts (as if it were written cheresh, and not choresh) of copper and iron.

Ibn Kaspi

It is like the first statement of Haman to his wife.