Although the name Satan comes from a biblical Hebrew word meaning “accuser,” this figure was very different from the leader of the forces of evil found in some post-biblical Jewish texts and in the New Testament. John Gregory Drummond explains:
In the Hebrew Bible, God’s greatest enemies are not fallen angels commanding armies of demons, or even the gods of other nations, but rather human beings. It isn’t the devil that spreads evil across the face of creation—it is mankind. Other than human beings, God has no nemesis, nor are there malevolent spiritual forces not under His authority. He is ultimately a God of justice. He is behind the good and the bad, behind the blessings and the curses. It is within this divine court of justice and retribution that Satan has his origins.
The Hebrew word satan, meaning “accuser” or “adversary,” occurs several times throughout the Hebrew Bible and refers to enemies both human and celestial alike. When referring to the celestial adversary, the word is typically accompanied by the definite article. He is ha-satan—the Accuser—and it is a job description rather than a proper name. From the Accuser’s appearances in the books of Job and Zechariah, it seems that his job entails calling attention to the unworthiness of mankind. The Accuser is essentially the prosecuting attorney of the divine court of God, and part of his job includes collecting evidence to prove his cases. With this bit of knowledge in mind, it isn’t difficult to envision the various “outcries against sin,” such as that against Sodom and Gomorrah (Genesis 18:20-21), as the voice of the Accuser.
It is [only] by the 1st century CE [that some forms of] Judaism developed a belief in the divine forces of darkness doing battle against the forces of light. This can be seen within the New Testament and other extra-biblical writings such as those found among the Dead Sea Scrolls. There are likely several factors that inspired these developments, including the influence of Persian, as well as Hellenistic, religions.
Read more on Bible History Daily: https://www.biblicalarchaeology.org/daily/biblical-topics/bible-interpretation/who-is-satan