Is it true that Lamech is not only not punished by God, he is allowed to boast (Gen 4:23-24)? Why?

It is true. After “my wounding” which was “to my hurt,” he has slain a “young man.” From the sound of this, it is at best self-defense, but it looks like manslaughter. It is entirely possible that God permitted this because it was in self-defense (which is permitted in the Mosaic code). But I suspect that, especially due to his self-comparison to Cain, this was a murder; moreover, it was not “an eye for an eye,” but rather it was a life for a “hurt.” But if it was indeed a murder, then why did God allow the crime to go unpunished? It seems that he had washed his hands of Cain and his progeny: he had turned his face from Cain, and unlike Seth, that line did not seem to be calling upon the name of the Lord. It is worth noting that not many generations would succeed this one before God would destroy all of humanity in the Flood. Lamech’s violence and arrogance well exemplify the “wickedness of man” that doubtless even by that time had become “great in the earth” (Gen 6:5). If Lamech was long-lived, then probably he was killed in the Flood.