This sort of thing has been tried before:

JSON.org License Literally Says it "shall be used for Good, not Evil": https://news.ycombinator.com/item?id=3693108

But, there is always the problem of defining evil: http://www.youtube.com/watch?v=JRxl02mULws

How long before the oath says "Except for IBM, its minions and customers..."?