We are in a near-future Earth. Humanity does not have access to Antimatter bombs and I forbid nuclear war.
What event could wipe out humanity within a week?
As one of my favorite source of one-liners would say: "large rocks landing where rocks ought not land."
It's actually really hard for us to end all of humanity within a week, if you define humanity as "all members of H. sapiens." Species are generally remarkably resilient, the Earth is a mighty big place, and a week is a very short period of time for anything like a virus to hit all of the corner pockets of society, leaving imact events as the most likely answer. However, there may be another option if all you seek is to destroy the humanity in us all, leaving our broken shells of a body in its wake: universal broadcast of cat videos. 3 days ought to do it.
Cort Ammon is right. It's incredibly tough to wipe out every single bit of humanity. People live in different environments, depending on different things in different ways. Not everybody has plumbing, or electricity, or wood. I can think of scenarios using those that could destroy all the people within a given area, but those scenarios wouldn't be applicable all over the globe.
So get rid of the planet - or at least change it into a hellhole where nobody can survive. This question and this question give interesting ways to do that. The best hard-science way is to have a large object impact the Earth. JPL has some statistics on impact probability. For cataclysmic impacts, the odds are low, but still nonzero.
Cliche, but needs to be mentioned. Just like in Terminator, artificial intelligence becomes conscious, wakes up, and for whatever *reason, decides people are a problem and cleans-house with nuclear fire.
Genuine mistakes by humanity include; giving it Internet access, building the damn thing in the first place.
*self-preservation, disgust, curiosity, paperclip maximisation.
There are tons of ways:
An asteroid: Obvious
A plague: Some people have ruled this out, because no plague on Earth, and probably no conceivable one, could spread throughout the human race in a week. But after all, an asteroid travels through space for who knows how long before impacting. If the events leading up to the event are non-lethal and unintentional, fair game, right? So the key is a virus that is passed from mother to child, is 100% lethal, with an incredibly long incubation period, one that is literally hard-coded into the virus and longer than human lifetimes, so that the life-cycle of the virus is essentially explicitly time-dependent. Obviously, no natural virus would have these characteristics. But a virus created by humans for an entirely non-lethal purpose, that mutated early in the gene line to be incredibly lethal? Possible, if unlikely.
Strangelet: Bad things come in small packages (https://en.wikipedia.org/wiki/Strangelet)
I wouldn't rule out plagues. Yes, no plague has ever been that catastrophic but that doesn't mean it's not possible. While no such thing could occur naturally (there's nothing close in nature) it could still be an accidental release of a bioweapon while it's developers are still working on a vaccine.
Think of a bug that doesn't kill in the usual way but instead causes paranoid schizophrenia and overdrives the heart. You have patients that resist treatment and flee (bringing the bug with them!) but which aren't so ill as to be incapacitated. They eventually die of a vascular rupture somewhere. It would also have to be something that spreads incredibly well.
Even more destructive would be a pathogen that drives it's victims to incredibly destructive acts. (Say, it leaves them believing everyone around is an enemy and that they only have a day or two to live anyway.) If you have a whole bunch of people trying to kill as many as possible you'll see a lot of death and a lot of destruction of technology. The two-man safeties on nukes mean no nuclear war (if everyone's an enemy they certainly won't be able to work with them to launch a nuke) but expect a lot of conventional weapons to be used and an awful lot of non-weapons. (Think of tens of thousands of Bophals. There will be a lot of engineers who understand how to create catastrophe from the systems they normally keep safe.)
As for other approaches there have been multiple mentions of impact events. However, a sufficiently large body can be deadly without it being an impact event. A neutron star or black hole could come traipsing through the solar system with very little warning. Imagine something coming in at a high angle to the ecliptic, for a neutron star the jets are aimed far from the Earth and for a black hole it can't have much of any matter along with it. A sufficiently near miss will do very bad things, the closer the pass the worse the Earth fares, for a very close pass the rubble that's left from passing within the intruder's Roche limit is ejected from the solar system.
A nearby supernova does not meet the requirements--there's nothing close enough that would fry us that badly that fast. Blowing off the ozone layer won't get the requisite kill.
The Grey Goo Scenario: Humanity creates self replicating Nanobots (to get rid of dust or an oil spill or something). The Nanobots get out of control and consume the whole world. Because it is an exponential process the Nanobots could destroy everything within a week. They might not leave behind much as a setting for a story though.
Back before they turned on the Large Hadron Collider, there was some concern that the experiments being run within it might generate a quantum black hole, which would then fall into the Earth and start librating around its core, eventually consuming the entire planet. Not being a physicist, I'm not sure how long this would take, but for purposes of fiction, a week doesn't sound too implausible.
The scientific consensus apparently debunked this possibility, saying that the LHC isn't powerful enough to produce such a singularity. However, some scientists hedged by saying such an event is merely "very unlikely" instead of flatly "impossible".
Of course, it's likely that sooner or later someone's going to build a more powerful particle accelerator, then another, then another. Will the scientific consensus ever shift? And if it does, will someone in the minority decide to turn the thing on anyway?
