The generation of the Flood earned their destruction through arrogance. According to Sanhedrin 108a, God gave them 120 years of warning. They spent those years mocking Noah.
The Sages taught that Noah would rebuke them, and they treated him with contempt. They said: "Old man, why are you building that ark?" Noah told them God was bringing a flood. They asked: a flood of what? If fire—they had a fireproof substance called alita. If water from below—they had iron plates to cover the earth. If water from above—they had a substance that could absorb it.
Noah replied: "If God wishes, He will bring the water from between your feet, and you can do nothing." The proof text is (Job 12:5): "A contemptible torch in the thought of him who is at ease, a thing ready for them whose foot slips."
The Talmud adds an explicit detail: "The waters of the Flood were as hard and thick as semen." The generation sinned through sexual depravity. The punishment matched the crime. Rav Hisda said: "With hot semen they sinned, and with hot water they were punished."
A question is raised about the seven-day delay before the Flood began (Genesis 7:10). Rav explained: these were the seven days of mourning for Methuselah, who had just died. God delayed universal destruction to honor one righteous man's death. Alternatively, during those seven days, God reversed the order of Creation—the sun rose in the west and set in the east—as a final warning. Another explanation: God gave them a foretaste of the World to Come so they would understand what their sins had cost them.
The generation's ultimate sin, according to Rabbi Yohanan, was robbery. They stole in amounts too small to prosecute—each person taking less than the minimum for legal action. No single theft was punishable, but the cumulative damage was total.