New York Congressman Daniel Sickles put it simply before killing a friend: "Key, you scoundrel, you have dishonored my house—you must die!" But the trial that followed was hardly simple and set a vast historic precedent when Sickles became the first American to successfully plead temporary insanity, Lapham's Quarterly reports. On February 27, 1859, Sickles gunned down his friend Philip Barton Key (whose father wrote "The Star-Spangled Banner") in Washington, DC, for having an affair with Sickles' wife. Then came the court case, where one of Sickles' seven lawyers claimed it was manslaughter, not murder, perpetrated in "a state of heat" that put Sickles above "the pale of accountability to the criminal law." He added that it's "no matter how a man becomes insane; is he insane, this is the question?"
The district attorney saw it differently, saying Sickles planned the attack and carried three pistols to the crime scene. But public opinion sided with Sickles, as did Harper's and the New York Times. And Sickles' theatricality won over jurors, moving some to tears when he cried during a friend's testimony and had to be escorted from the courtroom. The jury needed just 30 minutes to find him not guilty. In a twist, public opinion turned against Sickles when he made up with his wife, Teresa (who had only cheated on him over his "compulsive womanizing," according to an old Times review of a Sickles biography). And when Sickles became ambassador to Spain, after Teresa's death by tuberculosis, he supposedly hooked up with Spain's deposed queen, Isabella II—who was married at the time. (Read about the diary of a modern killer who's also using the insanity defense.)