People have become rebellious. Atum said he will destroy all he made and return the earth to the Primordial Water which was its original state. Atum will remain, in the form of a serpent, with Osiris.
Faulkner, Raymond (transl.). The Egyptian Book of the Dead, The Book of Going Forth by Day, Chronicle Books, San Francisco, 1994, plate 30.
(Unfortunately the version of the papyrus with the flood story is damaged and unclear)
Pharaoh Seti I’s funerary text tells of a sinful mankind being destroyed by a flood, for fighting and being generally evil. The Moon God Thoth was responsible.
"…Thoth, who had been seen by the Egyptians as the source of all their knowledge and science, had been credited with having caused a flood to punish humankind for wickedness. In this episode, related in Chapter CLXXV of the Book of the Dead, he had acted jointly with his counterpart Osiris. Both deities had subsequently ruled on earth after the human race had begun to flourish again."
Hancock, Graham, The Sign and the Seal. The Quest for the Lost Ark of the Covenant. Touchstone, 1993.
Tem, a creation God, 'was responsible for the primeval flood, which covered the entire earth and destroyed all of mankind except those in Tem's boat'.
Mercatante, Anthony S. (ed.) Encyclopedia of World Mythology and Legend, Child & Associates Publishing, Australia, 1988, p.613, 2008