How the mighty do fall!

[Hookbill the Goomba]

I have notised a theme that seems to run through a lot of Tolkien's Middle Earth. This is that People who were grate or mighty, are often those who fall into Evil. Take Saurman,

He was great once
Frodo Baggins, Return of the King, book six, The scouring of the Shire

Also, the Witch king of Angmar, was Grate, now evil and dead. Denathor, was a respectable leader, now a dead twisted old fool. Sauron, was a faithful servant of the Valar, now a dead Follower of Morgoth. Even Melkor, once the mightiest of the Valar, now a lonely, footless, evil spirit trapped in the Void.

Is Tolkien trying to say something here?