I agree that the "bad guys" in Tolkien's books tend to be copies of each other to some extent. But in Tolkien's defense, I would say that the same is true of such people in real life.
Think over the various dictators, organized crime figures, cult leaders, etc. over the last 100 years. They certainly represented (or at least claimed to) a wide variety of ideologies. But when you get down to it, they all are operating from the same script.
1. Try to persuade people to join/follow you if possible.
2. Coerce them to follow you, if persuasion fail.
3. If you can't coerce or persuade them, try and get rid of them, or at least cause them as much harm and misery as possible.
The other point is extreme selfishness. It's always about looking out for number one. They will work for someone else's benefit...as long and only as long as it coincides with their own.
Tolkien did conceive of a number of wonderful "bad" or semi-bad characters who were very interesting and individual. Feanor, Maedros, and Gollum are my personal favorites. In my view, the common element is that they all spend a great deal of their lives in an internal struggle with their own worst tendencies. Struggles which they ultimately lost, but at least ended up as much more interesting and sympathetic than those characters such as Morgoth and Sauron who just acted out their worst tendencies without any obvious attempt at self-control.
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