My thesis for my ten page paper is this: "The greatness of mankind can be overshadowed by their evil actions, which leads to corruption." Is it good?
This is a sound thesis statement for a ten page paper. If I were to quibble, I would say it would be better to change it into the following:
The greatness implicit in humans can be overshadowed by their evil actions, which leads to corruption.
This sounds better and "neater," in a sense, and also is not so much of a generalisation, as we can't describe all humans as being "great," unfortunately. This statement is a particularly interesting one and will require you to argue it by looking at cases from history or literature that shows the subversion of the potential in humanity and how it leads to corruption through evil deeds. There are a number of proofs you could use in this way, but I am thinking of Hitler and the Third Reich, which clearly had so much promise and potential, but was obviously fatally undermined by Hitler's own evil and corrupt nature.
This would just be one example. Either way, you have selected a very interesting thesis statement. Good luck!