[Gorwingel]

I guess you could sum up tolkien's message about this topic like this...

If you are mightly, powerful, and have great influence... don't become evil. Because it will just bring you death, failure, and dispair in the end.

All of the mighty who did fall in Tolkien's works, fell because they went to the evil side and strayed from their original purpose. They gave into their own personal greed, and desires for greatness and domination over everything. The mighty who didn't fall (like Gandalf) stuck to their original purpose, and kept on basically a path of "good". And of course they were rewarded in the end.