If we start a little smaller, setting our sights on just wiping out most current terrestrial animal life including humans, look to what happened with the pesticide DDT. DDT did not come anywhere near wiping out more than at most a few species, but it illustrates what could happen if we got unlucky (or someone deliberately set out to do maximal damage).
The problems with DDT were that (1) it could last a long time before breaking down, (2) it spread much more widely in the environment than was intended, and (3) it was not narrowly targeted to only affect pests.
When you are making a pesticide that is not narrowly targeted [1], you really want it to break down fast and to not spread much beyond where you specifically apply it.
Just make a pesticide that like DDT spreads far and wide and persists, but unlike DDT make it so one of the things it kills is nitrogen fixing bacteria. That would wipe out almost all of the base of terrestrial animal food chain.
[1] You make a narrowly targeted pesticide by basing it on the hormones that control the target's life cycle. You find some specific behavior of the target, such as its "mate and die" behavior, that is triggered by a specific hormone, and make your pesticide trigger that. Then all you need to do is apply the pesticide when the insect isn't yet sexually mature or when the weather is too cold for eggs to survive, and the insects mate and die, with no offspring produced.
There's an old science fiction story, where the aliens invade, not with flying saucers and ray guns, or hypervelocity near-C projectiles but by spreading what is essentially a pesticide globally that is tailored to affect human behavior and so nobody realizes what is really happening. There's no spaceship hanging around to hack into like in Independence day, there isn't even anyone willing to consider the idea that what is happening might be extrinsic.
"The tape recorder they put by his bed functioned right on through, but if anybody had been around to replay it they would have found little but babbling. “Gaea Gloriatrix,” he crooned, “Gaea girl, queen . . .” At times, he was grandiose and tormented. “Our life, your death!” he yelled. “Our death would have been your death, too, no need for that, no need.”
At other times, he was accusing. “What did you do about the dinosaurs?” he demanded. “Did they annoy you? How did you fix them? Cold. Queen, you’re too cold! You came close to it this time, my girl,” he raved. And then he wept and caressed the bedclothes and was maudlin."
The problems with DDT were that (1) it could last a long time before breaking down, (2) it spread much more widely in the environment than was intended, and (3) it was not narrowly targeted to only affect pests.
When you are making a pesticide that is not narrowly targeted [1], you really want it to break down fast and to not spread much beyond where you specifically apply it.
Just make a pesticide that like DDT spreads far and wide and persists, but unlike DDT make it so one of the things it kills is nitrogen fixing bacteria. That would wipe out almost all of the base of terrestrial animal food chain.
[1] You make a narrowly targeted pesticide by basing it on the hormones that control the target's life cycle. You find some specific behavior of the target, such as its "mate and die" behavior, that is triggered by a specific hormone, and make your pesticide trigger that. Then all you need to do is apply the pesticide when the insect isn't yet sexually mature or when the weather is too cold for eggs to survive, and the insects mate and die, with no offspring produced.