Not sure I totally agree with all the arguments here, but it's definitely food for thought.
In an essay "Notes on motives in the Silmarillion", published in History of Middle Earth vol. 10 Morgoth's Ring, Tolkien talks about this at some length. (Some of the other essays there such as "Melkor Morgoth" also touch on the topic.)
Essentially, Morgoth didn't immediately see himself as evil, during the primeval era of creation, but certainly by the time we get to the Flight of the Noldor, his only interests were in destruction and in conquest, the domination of the material universe and incarnate mortals. He definitely didn't have self-deceiving "falsely good" motives during the First Age.
Sauron, on the other hand, since his original interests were in "ordering" the world and since he originally worked for Morgoth's victory rather than for his own supremacy, seems to have had self-deceiving, "falsely good" motives far longer -- maybe even a mixture of good and evil motives during a portion of the Second Age. (Tolkien seems to imply that his repentance at the end of the First Age was partially genuine, though induced by fear, and he fell further into evil later, rather than purely dissembling.)