Portrait of a Poisoner

"Murder most foul," wrote Shakespeare of the poisoning of Hamlet's father, the King of Denmark, "most foul, strange, and unnatural." Even William Shakespeare might have trouble imagining a crime fouler, stranger and more unnatural than the Tylenol poisonings in Illinois. "This killer is so unusual," says Clinical Psychologist Samuel Roll of the University of New Mexico, "that our guidelines just don't work."

Who could be capable of such an act? What does the murderer hope to gain? What are the causes of such deranged behavior? These are the questions occupying police, psychologists and...