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.
The reference in Isaiah actually refers to Babylon (the “morning star” of civilizations) and predicts its fall. Just about any Biblical scholar will tell you that; it’s not even disputed. A medieval monk who was not particularly familiar with the context associated that phrase with Satan... but “Lucifer” was never intended (by the Hebrews) to refer to Satan.