OK, guys. I'm afraid I might be jumping into the deep end of the grammatical pool here but what the heck, I'll give it shot.

The original sentence: We know a successful deployment of a biological weapon, whether it's spread over a city or injected into our water,
will cause millions of deaths.
Expresses certainty in the outcome.We know a successful deployment of a biological weapon, whether it's spread over a city or injected into our water,
would cause millions of deaths.
Expresses certainty in the outcome.
We know a successful deployment of a biological weapon, whether it's spread over a city or injected into our water,
could cause millions of deaths.
Expectation but not certainty.
We believe that a successful deployment of a biological weapon, whether it's spread over a city or injected into our water, will/would/could/might cause millions of deaths.
Expectation but not certainty.