In Gen. Ch 4, we see the very first recorded murder in history with Cain killing Abel. What is interesting to note is the events that lead up to the murder. Cain and Abel both bring an offering to God; God has regard for Abel's offering but none for Cain's. It is clear from the text in verse 6 that at some point God had given Cain and Abel specific instructions about offerings and that Abel refused to obey those instructions. In verse 7 God warns Cain about the peril of sin and Cain obviously doesn't heed the warning, since in verse 8 he murders Abel and is therefore banished from Eden; Cain settles in Nod, a land ease of Eden. Ch. 4 then goes on to discuss various genealogies. A few interesting side notes on Ch. 4: It mentions the very first bigamous relationship in verse 19 and in the final verse of the chapter it declares "Then men began to call upon the name of the Lord" indicating that before this time men had not done so.
Chapter 5 discusses the descendants of Adam. JM notes that the phrase "And he died" is interesting since it is a reminder of the consequence of Adam's sin. A few points of interest in Chapter 5: Even after the curse of Gen. 3 man's lifespan was between 700-900 years. Enoch, son of Jared, did not die but instead was simply taken by God after 365 years.
Chapter 6 discusses the corruption of mankind along with Noah and the Ark. Firstly God notes that from this point forward, no man shall live longer than 120 years. Secondly, it notes how fallen angels (Sons of God) went into the daughters of man (human women) which caused them to have corrupt offspring (sometimes called "Giants" or "men of renown"). Thirdly, God saw that men were wicked and sinful and decided to blot out man from the Earth. Noah, however, found favor in the eyes of the Lord so God instructed him to build an Ark which was 450 feet long, 75 feet wide and 45 feet high. He told Noah to bring two of every animal, male and female, on the Ark with him. Noah did as God instructed.
