I heard a beautiful shiur on this topic in which Kayin was presented as a very misunderstood person. Though he did a terrible, horrible avera, he was also the first baal teshuvah. While Adam Harishon deflected his guilt, Kayin acknowledged his sin, saying, “Is my sin too great to bear?” In other words, he admitted he sinned.
As a consequence — and as part of his teshuvah — he was required to become a nomad. Therefore, it would make little sense for him to build a city, knowing he wouldn’t be able to enjoy it.
So why did he do it? Why did he a build a city?
The answer lies in understanding what a city is.
A city is a way to unify people, to bring them together in one locale. In order for a city to function, there needs to be a justice system.
Kayin, as the first murderer, brought death and division to the world — so he set out to rectify his sin by doing the opposite: unifying people through the building of a city, through causing a structure for a justice.
I believe this was from the sicha of the Lubavitcher Rebbe.