I'd like to reiterate this point for clarity, since I'm sure some people would say "no, it cannot be! What about Hitler. . ." It also begs the issue of moral relativism, that dead horse we keep beating.

A parable:

A medical student may or may not be murdered by a racist. This student, if he survives, will go on to discover hemoglobin. If he doesn't survive, others who would be saved by his discovery will have to wait until someone else discovers it. If he does survive, some people will die do to mishandleing of the research.

Circumstantially, the racist will kill someone. If it is not the medical student, it will be someone else. Because everyone is likely to save someone's life during their time on earth, chances are the racist will not only kill that person, but someone else, and so on down the chain of time. A small percentage of individuals are like the medical student, and will go on to save lots of lives.

So it is that the killing of even one person puts the lives of many others at risk, and yet may be of benefit. For sanity's sake, we choose to make choices based on immediate conseqences. You may see that, as hambydamnit said, it is impossible to perform a perfect ethical action without having both full knowledge and full control over reality.