We all long for the time when Satan and all evil will be destroyed.  We know his destruction is coming; you mentioned the "lake of fire," also known as the "second death," in Revelation 20:14.  This will be the end of Satan and all who will follow him at the close of the 1000-year reign of Christ.  (Rev. 20:6)  But God allows Satan to exist and to cause evil and suffering in the world until he is bound during Christ's kingdom on earth (Rev. 20:1-3).  Why?

Satan became the enemy of God when he, a beautiful angel in heaven (originally called Lucifer), decided he wanted to "exalt

[his] throne above the stars of God."  (Isaiah 14:12, 13)  He has not yet given up trying to usurp God's power. Many people ask, if God is all-powerful, why has He not taken Satan out of the picture already? It's because God has a plan.

Was it really only Satan's fault that the human race fell into sin and death?  Adam was given the command not to eat of one tree in the Garden of Eden, otherwise he would die.  It was not a difficult rule to follow.  But Adam's obedience depended on his utmost  respect for God as being all-wise and all-powerful.  Adam was created perfect.  However, he was inexperienced and had not known death and evil before he disobeyed God.  God didn't want a race of "robots" programmed to serve Him.  He gave man an intellect and free will.  God wanted mankind to worship Him "in spirit and in truth."  (John 4:24)  Our heavenly Father knew that that would be the only way for the human race to be happy and live in peace.  God wanted (and still wants) mankind to learn to love righteousness and to understand why obeying God is in their best interest.

There are two main ways to learn something.  One way is through instruction and example.  The other is through experiencing something.  When Adam and Eve were instructed not to eat the fruit of the one tree, they were easily led astray because they didn't really understand that God's way was best.  Now, after thousands of years of experiencing  sin and death, the human race is learning what misery comes as the result of disobeying God.

Let's imagine that God had prevented Satan from trying to get control of Adam and Eve.  They may have lived without sinning for some time.  Their children may have also lived a sinless life for years.  But somewhere along the line, it's inevitable that somebody would have become selfish and proud and thought he/she knew better than God, just the way Lucifer did.  (After all, the Bible does not say that Lucifer was tempted!)  That person might influence other people to join in his rebellion.  And those people would most likely have children, who would inherit sin and cause suffering in the world.  God would have to punish them with death.  But then how would God redeem them?  As it is, God sent His Son, Jesus Christ, to be a ransom for Adam (I Tim. 2:5,6), a perfect man for a perfect man.  The unborn human race in Adam was paid for in that one sacrifice.  "For as in Adam all die, even so in Christ shall all be made alive."  (I Cor. 15:22)  If God had permitted the human race to multiply before sin entered, redeeming mankind would have been much more complicated. 

The way God has planned it, now all in their graves will be resurrected.  ("For since by man came death, by man came also the resurrection of the dead."  I Cor. 15:21)  And mankind will come out of the grave with the experience they gained while living in a sin-filled world.  Those who have followed the Lord Jesus closely, even "unto death"  (Rev. 2:10), are promised a heavenly reward, where they will reign over the earth with Christ (II Tim. 2:12; Rev. 5:10).  But most of mankind will be raised to "a resurrection of judgment" on the earth, where they will learn righteousness.  (John 5:29 NASB, Psalm 96:11-13, Isaiah 26:9)  The experience with sin gained in this life will help people choose God's ways in the next life, which will lead to joy and peace in a restored, perfect earth.

In summary, God is permitting mankind to experience the results of sin. God could have destroyed Satan at the time of the flood, but God chose to continue to permit Satan’s usurped-dominion of the earth (2 Cor. 4:4) until Jesus reigns in righteousness on earth.