'Why have you done this terrible thing? Your brother's blood is crying out to me from the ground, like a voice calling for revenge. 11 You are placed under a curse . . . So the Lord put a mark on Cain . . . (Genesis 4:10 - 11, 15)
Why did God spare the life of Cain instead of taking it? Why did he even bother with a mark? One reason is that he had not yet given man a command not to murder another human. Such a command will be given, however, right after Noah leaves the ark after the floodwaters subside (Genesis 9:6).
An additional (and likely more important) reason Cain was given a mark and allowed to live is so that other humans would be warned of God's view and judgment on sin. If he was put to death as a punishment that death would soon be forgotten (Ecclesiastes 8:10) and the lesson of what happened lost. He is cursed, instead, to wander the earth with no permanent home. His constant wandering (due to being a fugitive), coupled with a mark he will soon receive, will be a living testament to others not to repeat the same mistake (Genesis 4:13 - 15).
Why would Cain, even before receiving the mark, greatly fear someone taking vengeance on him if, as some assume, no one else but Adam and Eve where living at the time? The Bible does not state Adam's age when either of his first two sons were born. Seth was born after the death of Abel when Adam was 130 years old. The youngest age at which the first patriarchs (Seth, Enosh, Cainan, and all those to Noah) gave birth to their first son was sixty-five.
If we, conservatively, assume Adam had his first son at 65 years old, then that would give Adam and Eve 50 years or more to procreate until Abel's murder. A footnote in the writings of the Jewish historian Flavius Josephus states a very old tradition that said Adam had a total of thirty-three sons and twenty-one daughters. The number of Adam and Eve's children, grandchildren, great grandchildren, etc. that existed when Cain received his punishment would have been enough to warrant his concern.
Why does God give a warning, before the mark is given, that if Cain is killed seven lives will be lost? It is because, in part, it would defeat God's judgment and the purpose why he was allowed the world's first murderer to live. God seems to have reserved revenge for his crime to himself, and therefore warned others they should not take to implement this revenge by their own authority.
The opinions and speculations about what was the mark have varied greatly throughout the years. Different people and groups have argued that the special sign God gave was a horn (either short or long), leprosy, Abel's guard dog, a special symbol or letter, or a letter used in God's name, a tattoo, dark skin (some feel he was turned black), His own name and even the rite of circumcision. Some think the Eternal turned him into a giant! The truth is the Bible simply does not clearly tell us what the unique mark of Cain was. Any conjecture, therefore, regarding this issue is ultimately based on opinion and not Scripture.