Today, the political world has been fascinated by Rand Paul's filibuster of the Brennan nomination to the CIA, over Eric Holder's weasly comments on whether and when drone strikes on US soil would be acceptable. While Ron Wyden is fighting the good fight, the GOP (or certain parts thereof) is making the Dems look foolish on this.
But there's a simple answer to the question. Never, unless the country is under military attack.
But there's a more important question which needs to be asked:
What about other means of killing?
The police--in every city and state--kill people (US citizens as well as foreign citizens of various immigration status) all the damn time. In some cases, such shootings are justified--a violent offender is clearly placing the public's life in danger, and ignores law enforcement commands to stand down. In many cases, though, such killings are dubious or manifestly UN-justified. The police--at all levels of law enforcement--are extremely overzealous in their use of deadly force, and frequently people are shot and killed by the cops who pose a minimal danger to the public, and/or are outright innocent. A big problem with modern law-enforcement is that they think and act like they are soldiers, and the citizens they are sworn to protect are subjects of an occupied state.
And in very FEW cases are cops prosecuted for this.
And another question: Does it matter if it's a cop with a service sidearm, or with heavier weaponry (including SWAT gear), or a government agent piloting a combat drone? Does it matter if it's sworn domestic law enforcement, the military, or intelligence?
Actually, it does--despite my stated reservations about cops killing people, it's still better than the alternative. Cops at the scene with side-arms are far less likely to cause indiscriminate mayhem then remote-piloted or auto-piloted machinery, particularly machinery armed with military-grade weaponry.
As far as the specific question on drones go--the only scenario in which I would consider violent use of a) drones, or b) military/intelligence personnel on American soil to be acceptable--at all--would be in repelling a military (or equivalent) invasion or attack, or similar circumstance, in which law enforcement is unable to respond with traditional tactics. The last time this circumstance occurred on US soil was 9/11--reportedly, jets were scrambled to shoot down Flight 93, though it crashed before the Air Force could get to it. Before 9/11, the last prior event that would qualify? Pearl Harbor. Before that? The Civil War. Before that? The redcoats marching on Washington DC in the War of 1812, and burning much of it to the ground.
And in these sorts of events, the citizenship of the perpetrators simply does not matter.
Anything else? No. Ordinary crime-fighting? No. Suspected terrorists or foreign agents on US soil (who are not engaged in an immediate attack to which law enforcement cannot respond)? No. Domestic insurrection or protest? No. Labor unrest? No. Organized crime? No. Bandits like Pancho Villa? No.
(Of course, the military has responded against numerous such events in our history, firing on civilians for things like demanding payment of army bonuses, or striking, or protesting taxes, or what have you. But that doesn't make it right or acceptable going forward).
It's a pretty easy question to answer. The longer it goes un-answered, the more ridiculous the Administration looks--and deserves to look.