Lamech the "Great" (in his own eyes)

Genesis 4:23-5:1

Lamech said to his wives, "Adah and Zillah, listen to me; wives of Lamech, hear my words.  I have killed a man for wounding me, a young man for injuring me.  If Cain is avenged seven times, then Lamech seventy-seven times."  Adam lay with his wife again, and she gave birth to a son and named him Seth, saying, "God has granted me another child in place of Abel, since Cain killed him."  Seth also had a son, and he named him Enosh.  At that time men began to call on the name of the Lord.  This is the written account of Adam's line.

This story is always offensive to me.  The fact that Lamech killed a man is bad, but I wonder if it was in self defense.  Up to that point, we don't know about this Lamech.  But Lamech speaks up, and is not afraid to place himself in the place of God in appropriating God's protection of Cain by Lamech's own word and not God's.  Not only that, but he places himself on a higher level of importance, declaring vengence to be seventy times greater.  

Of greater interest is the implication that God had favor on Cain, and did not necessarily vent his anger on him.  Sure, God drove him out, but what was the nature of the mark God put on him?  Whatever it was, Lamech felt it was positive, perhaps even protective.