Genesis 4:17 - City life

And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch.

Joel F. Drinkard, Jr., a professor of Old Testament Interpretation, states the following regarding cities:

“The emergence of cities often marks the move to civilization…. Most of the commercial activity for the region was carried out in the city. Usually, the city was located on the main highway or intersection of highways and trade route through the area. The major sanctuary or worship place would be most frequently located in the city, making it a center of religious pilgrimages and celebrations…. Among the chief concerns a city would be built at a particular location was the presence of food and water nearby, along with the raw materials for shelter, tools and industry “ (Holman Illustrated Bible Dictionary, Copyright 2003, Cities and Urban Life, pages 301-303).

Genesis 4:15 - Sevenfold

And the LORD said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the LORD set a mark upon Cain, lest any finding him should kill him.

Vengeance belongs to God (Psalms 94:1), however, sevenfold is also used in determining the penalty for thievery. Proverbs 6:30-31 states that "Men do not despise a thief, if he steal to satisfy his soul when he is hungry; But if he be found, he shall restore sevenfold; he shall give all the substance of his house."

Restoring seven times may seem like a hefty fine but not when considering the thefts that robbers get away with for "the tents of robbers prosper," Job 12:6.

Genesis 4:12 - Sin deminishes success

When thou tillest the ground, it shall not henceforth yield unto thee her strength; a fugitive and a vagabond shalt thou be in the earth.

God cursed the fruits of Cain's labor because Cain killed Abel, his brother. No matter how hard Cain worked the ground it still wasn't going to yield up to it's potential. This was also the case for Adam and Eve, Cain's parents. They fell from God providing everything, to having to work for everything. Sin is sin and it's all relevant.