Is God's plan of redemption dependent upon Satan's continued existence? Is this why Satan is still allowed to exist?
John 12:31
ESV - 31 Now is the judgment of this world; now will the ruler of this world be cast out.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Everything that exists came from God except evil. Lucifer created evil by defiance of God while in Heaven - "I will be as the Most High God" "I will exalt my throne above the Throne of God."
Sin followed mankind but was spawned from the rebel angel Lucifer and propagated by the 1/3 of the Heavenly Hosts/Angels who were under him and followed in his rebellion to overthrow God. Kicked to earth the agenda is to defy God still - a quest to be Lord of the earth and eliminate the godly, clearly revealed in what we see in the world today where rebellion and hate want to consume the world.
Adam and Eve fell and lost the clothing of the Holy Spirit when He departed them and they were left naked seeing all because the bright glory of God departed. Yet though they gave inroads through sin for Lucifer/Satan to have some dominion/control over them they regretted their rebellion instigated by Lucifer. They still saw God and knew He loved them but shame and hurt gave way for the one who caused the hurt and shame to lord over them in many ways - Lucifer.
Lucifer -Satan as God called him after Adam and Eve fell into sin- urges us to sin 'calling it cool' then when we sin he condemns us before God and mankind and gets us deeper into sin to control us. If he can he will convince us that it is OK to sin and enjoy the temporal pleasures of sin so much that we call evil good and good evil and there is not doorway back to God because God becomes evil and Lucifer (who feeds their lusts and greed) becomes good. This is the only people that can't be saved - though Jesus died for them and offers the covering of their sins to save them.
As for the source of good and evil - God can only do good. He NEVER CHANGES. He has repented of being frustrated with some but I know God well enough that He was looking for an intercessor to plead with them on behalf of those who were so evil and defiant of God and mankind. Sodom and Gomorrah is a good example - Abraham pleaded with God and got the number of righteous men down ridiculously low, bargaining with God. As low a righteous people in S&G agreed, there were not even that many so God went through with His plans to kill the cancer for humanity called Sodom and Gomorrah. God NEVER wanted mankind to rebel to the degree that He had to act against man. God wanted and wants all to be saved through His Son Jesus, the eternal Word of God Who took on flesh to take away the penalty of God and restore Adam's race to Himself in Heaven. No salvation for Nehpalim genetically impure creations.
As for Job, Job though righteous was not perfect by no means. "God gives and takes away" is not true. God gives and Lucifer/Satan kills, steals, and destroys. A quick thought, the fist couple of chapters deal with the story about Job as a foreword from the author, the other chapters are the story of old. The foreword tells abut Lucifer/Satan dealing with God as well as a quick intro to Job. The though, NEVER in the story of Job following is there ANY mention of Satan. "The Lord gives and takes away" combined with "what I so greatly feared came upon me" tell Job's personal story, not the narration. Fear tore down and destroyed while faith builds and heals. Job simply knew NOTHING about Satan and only could point to God, not knowing the enemy. Job did so well by not rejecting God and dying at the hands of his unknown enemy Lucifer/Satan.