What was so severe about the sin of the members of the generation of the flood? According to the talmudic Sages, the generation of the flood was guilty of many and various sins, but the sin that sealed their fate was robbery.

Come and see how powerful robbery is, as the generation of the flood violated every prohibition, but their sentence was not sealed until they extended their hands into robbery, as it is stated: “As the earth is filled with villainy because of them; and behold, I will destroy them with the earth” (Genesis 6:13). (Sanhedrin 108a)