@ARoyalPrincess the way the legendarium is built up the Valar are in likeness to Arch-angels and Maiar to Angels, that's probably what they meant.
Sauron was known as Mairon and was a Maiar in service to Aulë (where he learned crafting to a degree where he could eventually create something so mighty as the One). He wanted more than anything to create order in the world (and it's inhabitants) and eventually got so obsessed with the idea that he found his personal goals to be quicker reached if allying with Melkor. But he always held his own agenda first.
No price of order was too big to him, including enslavement and death. And he quickly realised that terror and fear were effective tools for quick results.
Sauron left Valinor to openly join Melkor when he established his first strongholds in Middle-Earth and became the commander of Angband.
