The Chicago Tylenol Murders
Setting the stage: The murders involved a series of deaths involving Tylenol laced with potassium cyanide due to drug tampering, and while there were prime suspects, there was never anyone charged, and it still remains unsolved.
These deaths were unconnected. Simply, it was a specific batch of Tylenol that was shipped out that had been laced with the poison, so whichever unlucky person picked up a bottle from this batch was inevitably doomed to death. Targets? There weren't any due to the random nature of the marketplace, which leaves a murderer and a motive to solve.
Perhaps this was birthed from the need to make a statement. There are some who openly oppose the use of pills and supplements to aid in human health and wellness, opting for natural ways of healing and pain prevention. Perhaps this person in particular held this type of belief in high value, and wanted to deter the populous from consuming product that they openly rejected. What better way to deter the use of pills than to poison them and create a false correlation between the use of pills and death? Furthermore, Tylenol is one of the most popular painkillers and over-the-counter pills on the market, which means the tampering of such a prominent pill would create lasting after-effects.
Of course, with laboratory tests, one could easily deduce that a batch was tampered with by a third party, but would the average consumer be aware of this? It's similar to the reaction when there's a recall on lettuce due to an outbreak of E.coli in grocery stores; people won't want to purchase lettuce, or will be hesitant to, if they know there's an outbreak going on.
There are caveats, but the motive still stands.