I think that Sauron wasn't just a mindless slave to Morgoth as mentioned by the poster, Sauron was evil because he was in awe of and wanted to be exactly like Morgoth, a kind of hero worship if you will. Also, I don't think the Valar would have allowed Sauron to completely conquer Middle Earth. If Sauron managed to destroy all resistance to himself I think the Valar would be obligated to oppose his force with theirs like in the War of the Wrath. They would not allow anyone to completely control the Children of Illuvatar because of their loyalty to him.
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His sword was long his lance was keen
His shining helm afar was seen
The countless stars of heavens field
Were mirrored in his silver shield
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