I was thinking this as well. If a new drug works well for 99% of the people, has mildish side effects for 0.9% and is really bad for 0.1% of people, that's no good. But if a nerve agent kills 99% of people and is not effective on 1%, that's just fine.
Also, compounds that are fatal tend to be so to all life forms vs just humans with the variation being dosage. It goes without saying the odds of taking a compound and finding a drug that does one very specific thing without doing anything else.. and demonstrating that it's safe in people, is orders of magnitude less likely than finding a compound that is lethal.
