When I think to scenarios that no human would survive, I think of the end-Permian mass extinction event, which wiped out most complex plant and animal life in both land and sea.

One speculated mechanism for this was a mass release of hydrogen sulfide gas from the oceans, which is acutely toxic. Not only does this kill most air-breathing life, it also strips the ozone layer and irradiates the surface. The planet is then left to cook in this manner for some centuries.

Engineering an event like this would require immense industrial capacity, as well as a deliberate objective of wiping out humanity. But I don't think it's beyond our ability, if we were both clever and stupid enough to try it. There are likely chemical compounds that would do the job more efficiently than hydrogen sulfide.