This is a small but critical part of my story; I need to kill off most of humanity (leaving a 5- or 6- digit number of survivors) and render the planet uninhabitable long-term, but survivable for a short time (as short as a few hours is OK). So I've been thinking about this scenario:
A large comet/asteroid impact turns one whole side of the planet to lava; everyone on that side is vaporized instantly and the atmosphere is pushed into space.
On the downstream (non-impact) side, there are earthquakes that would register 12-15 on the Richter scale, if the Richter scale went that high (I get that it's logarithmic). In addition there are tornado-force winds as the atmosphere rushes to fill the void on the upstream side of the planet, and torrential rainfall as water condenses out of the suddenly rarefied air.
Most people on the downstream side die from injuries and there is only a short time before the air pressure drops to unsurvivable levels and temperatures become lethal due to equalization with the lava on the upstream side.
Does this make reasonable sense, physics-wise? It doesn't have to be perfect since most of the story takes place afterwards and not on Earth (and we're not coming back), so we don't have to worry too much about details.