The Torah says that the generation of the Flood was destroyed because it was corrupt. The term the Torah uses for "corrupt" is chamas. The Talmud definition of chamas is taking something from another person by force, but paying for it. This is a type of thievery which a person may justify because, "I paid him for it." It is thievery nevertheless because the person did not want to sell it.
An ordinary thief may do teshuva because his conscience may bother him. A chamas thief will never do teshuva because he rationalizes his behavior and thinks he did no wrong. The generation of the Flood rationalized all their wrongful acts, hence they were beyond teshuva and had to be destroyed.
We have extraordinary ability to rationalize. We must be on our guard that we do not delude ourselves.