Imagine we have two sleeping people. One is dying and needs medicine. The other is healthy. The first dies because a person nearby who is capable of giving her a life-saving drug does not do so. The other dies because another person injects them with poison. Which is more in the wrong, the one who failed to give the medicine, or the one who injected the poison?
This is the "omission bias," which is the tendency to consider harmful actions as worse than equally harmful inactions. It's debatable, morally, whether this is a bad bias or not. That is, some think it should not be considered a bias at all.
I want to talk about it in terms of governmental drug approval. Kurzweil suggested in his The Singularity Is Near that people are loathe to approve drugs that might kill you. The way he sees it, (and I'm sympathetic to this view) is that people think that giving someone a drug that will kill 20% and save 80% of terminally ill people should not be put on the market. I see this as omission bias at work. People think that letting people die because they did not get a drug is better than killing someone by giving them a drug.
Accepting the omission bias as a bias to be overcome, however, has some disturbing ethical results. Is there anyone on Earth that has ever died because of your inaction? Let me rephrase that: are there any actions you could have taken in the past that would have saved anyone at all? People who could not afford medicine? Is there anything you can do now that would save the life of anyone in the future? If so, and you believe that omission bias is actually an illegitimate bias, then you're as guilty as a murderer.