Does the Devil work for God? Short Answer: No. Long Answer that will require explanation: Yes.
The Devil does not work for God in the sense that the Devil hates to do so; he sees God as an enemy so his plans and actions are motivated to destroy or thwart or at least delay God’s plans and purposes.
However, because God is in control and accomplishes His purposes and that includes overruling the Devil’s actions, indirectly the Devil serves or works for God. To be a little more technically accurate, the Devil is a tool that is being used by God to accomplish His purposes.
Let us take the greatest ‘murder’ in all of human history as an example. (I put quotation marks for murder because it was not a murder after all, for Jesus voluntarily gave His life – John 10:11–15). And that is the Death of Jesus.
Who killed Jesus? The Jews? Pontius Pilate? Satan? The Pharisees? Well, they all had their part in killing Jesus, so partially they all did kill Jesus, they contributed to His physical earthly death.
However, it was God Himself who killed Jesus. It all started when God spoke of it in the Garden of Eden:
Genesis 3:15 (KJV) And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.
That Scripture tells us that it was God who dictated what will surely happen in the future at the Cross: The Messiah would physically die as an Offering to God for man’s salvation, and thru that death Satan would be permanently defeated. That’s it. God ordained the death of His Son and the Son agrees to the plan of the Father:
John 10:11, 15, 17-18 (KJV) I am the good shepherd: the good shepherd giveth his life for the sheep.
As the Father knoweth me, even so know I the Father: and I lay down my life for the sheep.
Therefore doth my Father love me, because I lay down my life, that I might take it again.
No man taketh it from me, but I lay it down of myself. I have power to lay it down, and I have power to take it again. This commandment have I received of my Father.
Those Scriptures tell us plainly that it was not Satan who took away Jesus’s life – but it was the plan of the Father and the Son for the Messiah to be killed. It might seem brutal to us, but it was the Father who killed the Son. Again, another Scripture:
Acts 2:23-24 (KJV) Him, being delivered by the determinate counsel and foreknowledge of God, ye have taken, and by wicked hands have crucified and slain:
Whom God hath raised up, having loosed the pains of death: because it was not possible that he should be holden of it.
So, in that incident, it was God’s plan to offer the Messiah as the perfect Lamb who takes away the sin of the world (John 1:29), and the Jewish leaders and the unbelieving Jews and Satan took their respective parts in fulfilling it. So indirectly, the Devil did work for God.
Another example is the popular suffering of Job. Satan wanted to destroy the faith and devotion of Job to God, yet God wanted to strengthen Job’s faith and to show to Satan that Job loves God above all else, not because He is blessing Job. So again Satan was just a tool.
The more the Devil rebel against God, the more of God’s Justice and Holy Wrath will be magnified; the more people sin, the more God’s Grace of Forgiveness and God’s Righteous Judgement will be glorified.
Everything has been directed by God, and in the End everything and everyone will have their respective places as determined by God as revealed in the Bible.