From the article:
The paper’s basic contention is that the government has the authority to carry out the extrajudicial killing of an American citizen if ‘an informed, high-level official’ deems him to present a ‘continuing’ threat to the country. This sweeping authority is said to exist even if the threat presented isn’t imminent in any ordinary sense of that word, even if the target has never been charged with a crime or informed of the allegations against him, and even if the target is not located anywhere near an actual battlefield. The white paper purports to recognize some limits on the authority it sets out, but the limits are so vague and elastic that they will be easily manipulated.
This speaks very concisely about the difference between governments and private citizens in using offensive capabilities. The imminent threat concept speaks to the immediacy of an event that will potentially result in the loss of life or property. By removing immediacy or imminence from the equation, this moves the issue into the realm of retaliation, which is never legal for a private citizen.