My take on this: it's already hard enough not to screw the world any more that it already is. My favorite example: adding "[this project] shall be used for Good, not Evil" to your code will bring infinite arguing back and forth[1].
I try to focus on the small things. There are alternatives to owning the Bill & Melinda Gates Foundation. Choosing not to work for Zynga, or refusing to help the NSA's data collection is more than enough for a start. Write an app that makes life in the 3rd world easier, and you are already ahead of the curve.