Tamweop said:
But nothing bad happens to him, God let him be. Is that to teach Christian that God is unpredictable?
The usual expectations of God's judgements are wrath and punishment. In fact, patience and mercy are more the ways of God.
The consequence for the lineage of Cain was already spelled out when Cain no longer walked in the presence of the Lord. It follows that anyone that follows Cain's path no longer walk with the Lord either.
Not so much a punishment then, but Cain's banishment was a consequence of his turning away from what God was telling him.
And Lamech had already been born into
that consequence of being a lineage outside of the Lord's presence.
What God offered to Lamech was really the same as he offered to Cain. This was a mark to protect him from others taking vengeance against his murder. To maintain a semblance of order, his murder becomes protected, lest the society degenerate even further into an orgy of blood feuds.
And in a sense, God's worst 'punishment' takes the form of this protection. Where murderers are protected, their punishment is that they must live their lives out in a society with murder as its basis.
Violence becomes the very foundation of their societies. Their socieities are not held together by the bonds of brotherly love, but by power relationships.
And just as those who 'live by the sword, die by the sword', violence and threats of violence become their constant companions.
Note also that Cain and his descendants are noted as founders of the city (ie civilisation), and the arts and technology that arise out of civilisation. Civilisation itself then, arises out of the ongoing protection of murderers.