Rev 12:9 And the great dragon was thrown down, the serpent of old who is called the devil and Satan, who deceives the whole world; he was thrown down to the earth, and his angels were thrown down with him.
You can understand my confusion.
The OT never equated Satan with the devil; it's a "deuterocanonical" concept that was incorporated into the New Testament by the Apostles because they used the Septruagint.
The Jews didn't associate evil with Satan. The Christians do. The roots of that are in the 'deuterocanonical' books. Judaism considers Satan a loyal and obedient servaht of God. Ihe is portrayed that way in the OT.