What is the cost of doing something versus doing nothing? Who has the responsibility to act?

As a human being, I believe that everyone who is anyone has the responsibility to act. If a nation goes into a genocide, with the intention of bringing it to an end, lives may be lost, but eventually, and usually quickly, the genocide will come to an end. Yet if a nation decides to do nothing when a genocide occurs, sure their own citizens will not be harmed, but the genocide will continue, until its objective is reached. And if their objective is to murder every Tutsi in Rwanda, the Hutu will not stop until every one of the hundreds of thousands of Tutsis is dead. Think about it, hundreds of thousands of Africans, or one hundred Americans? They are all humans.