Thinking, a book by Hardman and Macchi, says people would much rather let bad things happen, than take a risk to stop them:
Decisions with identical outcomes are judged as worse when they result from acts of commission than acts of omission. For example, most people are reluctant to vaccinate children against a potentially lethal flu when side effects of the vaccine can cause death. Faced with a flu epidemic that is expected to kill 10 out of 10,000 children, most people are not willing to accept a 9 out of 10,000 risk of death from vaccination, and the median acceptable maximum risk is 5 in 10,000. Apparantly, killing a child with a vaccination (that is, an act of commission) is perceived as worse than causing its death by failing to vaccinate (that is, an act of omission).