isn't lamech twisting God's word to cain just in order to justify his wrong-doing?...

Lamech had no fear of the Lord. He was boasting about killing a man for wounding him. He had no regrets for his deed and he boasts he was better than his forefather Cain who killed his own brother whom he had no reason to kill where as he had killed someone who hurt him. He thinks if God protected Cain from anyone who wanted to kill him, surely he is in a far more better position to receive God's protection from anyone who wanted to kill Him. He did not realise that Cain got this protection from God by asking for it. Lamech did not even mentions God and he just takes it for granted that much more protection will be there for him even without asking for it.
We see that nothing more is mentioned about Lamech or his generation.The history of Cain stops with Lamech. But the lifeline of the peopleof God exist for generation after generation. God is against those who are boasting about their sins, whereas He is merciful towards those who humble themselves and plead for His mercy. There is no end to His mercies.
we know that the salvation and forgiveness of sins exists for all kinds of sinners through the death of our Saviour on the cross. But only if we confess our sins to God with a true repented humble heart we can receive this forgiveness. If we are too proud to ask for forgiveness for our sins then we shall be far away from this salvation.
Let us not ruin our lives by thinking that we are far better than many others who are doing this and doing that and yet seem to be going on without any rebuke from God.Let us be doing what is good in God's sight.