Early Judaism did not have an “evil” force (i.e. the “devil”) that struggled against God, primarily because in it God was seen as opposed by the other gods of the heathens surrounding the Israelites (the Mosaic admonition to have “no other gods” before him was most likely a literal statement. Many parts of the Old Testament read like the other gods are both real and in conflict with the God of Israel). As such, Satan (literally “the accuser”) in the Old Testament was God’s attorney general, the subordinate power tasked with bringing the transgressions of God’s people to his attention.
By the time of late antiquity, the Hebrews of Israel had been exposed to various Eastern/Persian philosophies (through the Roman occupation) that gained popularity in the hellenistic world previously. These philosophies gave rise to religions of dualism (culminating in Manicheism shortly after Christ), and the Jews of Jesus’ time had been heavily influenced by these ideas. Between the time of the OT’s writing and the time of the New Testament, Satan had evolved from a servant of God into a dualistic opponent (but without the equal power that most dualistic or Manichean deities have). So the Satan of the New Testament is not easily reconciled with the story of Job.
The problem is that the Bible exists to teach wisdom and God’s will, and modern readers expect it to be a logical narrative like a history text-book. This leads to various intellectual gymnastics to make various parts of a document written over millennia fit seamlessly (to our eyes), rather than an appreciation of Job as God’s promise to never desert those who obey and love him.
The word “devil” is derived from Greek “diavolos”, which also means “accuser”.
Satan was always an evil entity in the Old Testament right from his appearance as the serpent in Genesis 3, although not consistently called by that name. In Isaiah 14:12, he was called “Heylel ben Shachar” (literally “bright [or howling] one, son of the dawn”; translated into “Lucifer qui mane oriebaris” in the Vulgate); in Ezekiel 28:12, he is called the “king of Tyre” (note the attributes that follow). There has been no change.