Why did not God destroy everyone when He flooded the earth, thereby ensuring a complete cleansing of sin. Why throw Satan and his angels out of heaven and thereby leaving them to perpetuate sin?

When God cursed Satan in Genesis 3:14, “the Lord God said unto the serpent, because thou hast done this, thou art cursed above all cattle, and above every beast of the field; upon thy belly shalt thou go, and dust shalt thou eat all the days of thy life:  Dust (or death) would be his food”.

According to Isaiah 14:12-15, Satan was formerly a day star who was “…cut down to the ground, you who laid the nations low! You said in your heart, ‘I will ascend to heaven, above the stars of God, I will set my throne on high… But you are brought down to Sheol to the far reaches of the pit.”

Ezekiel 28: 12-15,  “…Thus says the Lord God: “You were the signet of perfection, full of wisdom and perfect in beauty. You were in Eden, the garden of God…On the day that you were created they were prepared. You were an anointed guardian cherub. I placed you; you were on the holy mountain of God; in the midst of the stones of fire you walked. You were blameless in your ways from the day you were created, until unrighteousness was found in you.” Jesus stated Luke 10:18, that he saw “Satan fall like lightning from heaven.” Satan and his minions have been allowed to test mankind, (as permission was given to Satan to test Job in Job 6:1-22) in order to help them discern between good and evil, obedience and disobedience.

In contrast, according to Genesis 6:9, Noah was a righteous man, blameless among the people of his time who walked with God, in powerful contrast to the wicked lives of those referred to in Genesis 6:5. God had a plan to start over with righteous Noah of pure Adamic lineage to fulfill His promise that the seed of Eve would “bruise” (crush) Satan, Genesis 3:15. Noah’s family was commanded to multiply and fill the earth in Genesis 9.

We are heartened by the promise in Revelation 20, that in the future, “I saw an angel coming down from heaven, holding in his hand the key to the bottomless pit and a great chain. And he seized the dragon, that ancient serpent, who is the devil and Satan, and bound him for a thousand years, and threw him into the pit, and shut it and sealed it over him, so that he might not deceive the nations any longer, until the thousand years were ended.

During the thousand-year reign of the returned Lord Jesus, each individual will be subject to the same test that Adam experienced and will learn obedience, this time with experience.  We are promised that all but the unwilling will obey!