I found a commentary by Buddhist chaplain Mikel Ryuho Monnett that ties together two recent themes on this blog -- Jataka Tales and the death of Osama bin Laden. I found a slightly different version of the same Jataka Tale by Stephen Jenkins at The Guardian.
This particular Jataka Tale is one I've heard before, but I've been unable to find exactly where in comes from. If anyone knows, do speak up. But here it is --
Once the Buddha was a ship's captain, and he learned that one of the passengers planned to kill the other passengers. He thought that if he warned the other passengers, they would get angry and kill the homicidal passenger in a fit of rage, and this would create all manner of unfortunate karma for them. So he killed the would-be murderer himself, but he did so without anger.
The point of this is either that the Buddha took the unfortunate karma upon himself to spare his passengers, or that he generated merit by acting with compassion. Versions of the story seem to differ on that point.
Buddhists through the ages have debated whether it's morally acceptable to kill one being to save the lives of many, or whether the dharma requires us to stand aside and let events take their course rather than break the First Precept. My understanding is that Theravada tends to come down on the "hands off" side of the argument, whereas Mahayana takes a more interventionist position.
The tale about the captain might be construed as a permission slip to go about killing "bad" people. However, dividing humanity into "good" and "bad" is an act of vast ignorance. And it should go without saying that if the act is contaminated by the least bit of greed, hate, or ignorance, there will be unfortunate consequences to the do-er.
I see that Stephen Jenkins contributed a chapter to that dreadful Buddhist Warfare book edited by Michael Jerryson and Mark Juergensmeyer, although I don't remember what he wrote. But he makes a valid point in his Guardian piece that Buddhism on the whole does not take an absolutist position on not killing.
Our intentions, motivations, frame of mind, understanding, etc., combined with specific conditions may present us with a circumstance in which killing is the most compassionate option. The most common circumstance I can think of might be the animal shelter employee who is required to euthanize an animal that is too aggressive to be adopted. The alternative would be to keep the animal confined to a cage for the rest of its life, which is cruel.