Please do not insult me, Mr. Tanaka. [Kaoru's own expression becomes grim; it's easy to tell what Gundam thinks by that look alone.]

It is possible that they do not know what poison they choose. Miss Aozaki's case, for example; the poisons were unlabeled. If she had full control over those deaths, knew the exact affects, she surely would have come back to clean up the vomit or waited the proper time before attacking Miss Belarus instead of arriving early. There is also, of course, the possibility that vague descriptions are given and one was simply chosen because it seemed the most "interesting".