Morgoth is NOT an allegory of
Satan or any other evil
deity which one could equate with him. He is
Prof. Tolkien's personification of
evil. Tolkien has
denied and denied again that
anything in the
Lord of the Rings or the
Silmarillion is representative of
anything in the real world; it's not
World War II, the
Shire is not England,
Gondor is not France,
Morgoth is not Satan, the
One Ring is not the
Atom Bomb, and
Long Bottom Leaf is NOT marijuana.
Tolkien created these worlds as an environment for his languages to live in, and greatly supported the idea that he was just a historian rewriting ancient tales of mythology (The Red Book of Westmarch).
In short, yes, Morgoth could be EQUIVALENT to a Satan, but should never be seen to represent Satan.
This node does a very good job at looking at the subject in depth.