Sauron is a Maiar who became Melkor's satellite, drawing heavy influence from the Valar who directly opposed Eru who is essentially God. Melkor grew a deep hatred for all that Eru created and sought to destroy it without purpose, but Sauron saw purpose and wished to usurp God to create utopia. Sauron obviously is no where strong as the OG bad guy so who employed trickery to bend the will of others to suit his needs, the most well know method being that of the rings which Celebrimbor helped forge. As long as free will reigned in the world, Sauron's plans for utopia would never succeed. By the time Lord of The Rings happens, most of Eru's influence over the world had faded. The idea all along was to have man inherit Eä, the world, because of their meek behavior while the Valar and their descendents would always have higher ambitions that would inevitably bring about cataclysm to the world time and time again. Most of the higher beings already did not like this and Sauron especially. Him seeing this golden opportunity to regain his corporeal form and conquer Middle Earth during the Third Age set forth the events for Lord of The Rings.

Melkor is basically Satan and Sauron is Hitler. There you have it.