Can runaway methane gas emissions be an extinction level event?
IF, and it's a very big IF, all the methane hydrate were to be released rapidly, then the planetary climate would be stuffed. But it won't happen.

How much methane hydrate will be released due to global warming? Run 30 different atmospheric-ocean-coupled global climatic models, and you will get 30 different answers - all of them based upon arguable assumptions. The scaremongering doomsayers love to fanfare the worst case. In which case, some of your dire forebodings may come about. But first, take a balanced view of the physical reality. The vast majority of all the world's methane hydrate is sufficiently deep, on or in the sea floor, that it will be millennia, if ever, before they melt, thereby contributing to global warming. The deep ocean floor is everywhere only a few degrees above freezing, whereas almost all of the oceanic warming is in the uppermost few hundred metres. Check out the diagram in http://worldoceanreview.com/en/wor-1/ocean-chemistry/climate-change-and-methane-hydrates/ and you will see that the pressure-temperature control is such that most of the methane hydrate will always be stable. 

The current consensus is that some methane is already being emitted from the thawing tundra, mainly in Siberia, Canada and Alaska, and some more will be released from shallow seas as and when they warm up. But this won't dominate the global warming process. In fact methane release is just one of at least two dozen feedback processes, some negative but mostly positive, which will influence the rate of global warming for the next few centuries. 

Making the world 'toxic', or 'like Venus' or the 'atmosphere turning brown', or 'poisoned birds dropping out of the sky' are all absurd, but that's no reason for complacency. The rise in global atmospheric methane amounts to about 6 parts per billion per year since 2006, or slightly more than 6 ppb in polar regions; significant, but not at all catastrophic. I am far more worried about the accumulating global CO2, which still shows no sign of slowing down despite a dirge of conference resolutions, ineffective carbon taxes, and empty political rhetoric. 

As for an 'extinction event', the extreme rapidity of global warming, habitat loss, sea level rise, and ocean acidification, all adds up to the fact that we are already in an extinction event - not for humans, most of whom will be able to engineer their way through a 3 to 4 degree C global temperature rise, but for the rest of the biosphere - with, or without, extra methane. 

Notwithstanding the scientific facts, maybe we can all relax, because Donald Trump assures us that climate change is just a fictitious Chinese conspiracy.