[LillyPlay]

You know they're going to bomb a school.

That school just happens to be your daughter's school.

Do you wait for the attack to happen before you kill them?

Is there a clear and present danger?
Are they crossing the schoolyard with explosives, while I just happen to be there holding a loaded gun?
Or is this a conversation I overhear between the waiters at a middle eastern restaurant?
If it's situation where there is time to inform the authorities, then inform the authorities. If there's a clear and present danger, take action to prevent it.

But if you are going to shoot the waiter when he brings the falafel because you think you heard him talking a blowing up a school, get a good lawyer, and prepare to spend a non-trivial amount of time in prison.