Ebola is one of the deadliest viruses. Its kill rate is only about 90%. Most biological agents would be less lethal. Delivery of biological agents on the sort of scale that could threaten the human race requires at the very least missile delivery (countering missiles once they've launched is hard), and then it's dicey: there's so many pockets of low-density population, and if you decide not to target a country, that country can very easily close its borders to protect itself (cf. the Ebola epidemic).
It is much, much easier to kill off the human race using nuclear weapons. And I'm not sure that's possible.
I'd note a simple possible solution to the Fermi paradox: A physics experiment that is logical to do, will discover new physics -- with really big and unexpected results. Or something like that. It seems something kills (or hides) civilizations.
The weaponized diseases were worse than anything natural (e.g. optimized to get a really toxic immune system reaction and so on). And multiple ones could be used at the same time, with different spread mechanisms and incubation times.
But about Ebola -- I have no clue if the kill ratios can be engineered to not go down for a fast spreading disease. (And if I did, I would not be able to talk about it, of course.)
Those militarized diseases are easily enough for an economical (/civilization) collapse, that would take the world at least generations to recover from. Which is bad enough, but is no extinction of course.
I don't know if nuclear weapons are on the table again, despite that they are spoken about in Russia -- it is probably just psychological warfare (maskarovka), everything new from there seems to be a smoke screen.
But how hard would it be for Iran or North Korea to build a Cobalt bomb, the doomsday machine out of Dr Strangelove?
It is much, much easier to kill off the human race using nuclear weapons. And I'm not sure that's possible.