[Zigûr]

"Sauron desired to be a God-King, and was held to be this by his servants." (Letter 183)
You say Sauron was looked on as a God in Númenor, but Professor Tolkien explicitly tells us in Morgoth's Ring that he wasn't. In Númenor he was, regardless of how powerful he became, still a prisoner and hostage, and only the representative of their new "god" (Morgoth). We have to factor in Sauron's pride. He could never be King or God in Númenor the way he was in Middle-earth, and I think that fact frustrated him to no end. I think ruling from behind the throne was never really good enough for him. We know that in the Third Age the remaining Black Númenórean societies did revere him as a God - after their old king and realm had been destroyed. I think this was the only way for Sauron to accomplish what he truly desired. And as Professor Tolkien points out, he had been partially (but not completely) infected by Morgoth's nihilism - wishing to destroy what he could not openly rule. Sauron was, I believe, in many ways an idealist (his largely all-or-nothing scheme with the Rings of Power, for example, a potential masterstroke which failed to ensnare anyone but Men, leaving him still vulnerable rather than completely in control) and didn't always behave in a completely pragmatic or necessarily even rational manner regarding his plans and ambitions.