Comment by philipkglass

A successful drug candidate must be useful in the treatment of human medical problems and not have harmful side effects that outweigh its benefits. A weaponized poison may have any number of harmful effects without diminishing its utility. A compound with really indiscriminate biochemical effects, like fluoroethyl fluoroacetate, makes a potent poison without any specific tuning for humans. It's much easier to discover compounds that genuinely harm people than those that genuinely help them.