A man named George Atzerodt was supposed to kill Johnson, while another named Lewis Powell (a.k.a Payne) was tasked to kill Seward. Atzerodt lost his nerve, but Powell stabbed Seward though did not kill him. Under the succession law of the time, Seward wasn't in line, but no doubt he was chosen to die because of his importance to the Lincoln administration and the consternation it would have caused. President Pro Tem of the Senate Lafayette Foster of Connecticut would have become acting president had Lincoln and Johnson both died. Sen. Foster remained second in line to the presidency until he lost his seat in 1867.
Lincoln's death in all its unfortunate detail -- Ford's Theater, Our American Counsin, "Sic semper tyrannis!" and all the rest -- is well known even to a nation generally uninterested in its history. Far less known is the story of the other victim of the plot that night, William Seward. (Below is the attempted murder, as depicted in Harper's Weekly.)
Douglas O. Linder writes at the "Great Trials" web site of the University of Missouri-Kansas City School of Law, "The conspiracy began to unfold around eight o'clock on April 14, when Powell met with Booth, who gave him weapons and a horse. At ten o'clock Powell and David Herold arrived at Seward's home in Washington. Powell told the servant who answered the door, William Bell, that he had a prescription for Secretary Seward from his doctor. Over Bell's objections, Powell began walking up the steps toward the Secretary's room, when he was confronted by the Secretary's son, Frederick Seward.
"Seward told Powell he would take the medicine, but Powell insisted on seeing the Secretary. When Seward resisted entry, Powell clubbed him violently with his revolver (fracturing Seward's head so severely that he would remain in a coma for sixty days), then slashed the Secretary's bodyguard, George Robinson, in the forehead with a bowie knife. Finally reaching the Secretary in his bed, Powell -- shouting, 'I'm mad, I'm mad!' -- stabbed him several times before he could be pulled off by Robinson and two other men.
"Powell raced down the stairs and out the door to his one-eyed bay mare. Attempting to flee in the direction of the Navy Yard bridge, Powell instead made a wrong turn and ended up spending the night in a cemetery near the Capitol.
"Powell was arrested on April 17 after he showed up at Mary Surratt's home with a pick-axe while she was being questioned by a party of military investigators. Powell -- at the unlikely hour of eleven p.m. -- claimed to have been hired to dig a gutter. Mary Surratt refused to back up his story and he was arrested on suspicion of his involvement in the assassination plot. When William Bell identified Powell as Seward's attacker, Powell laughed. Further confirmation of Powell's guilt came in the form of blood spots found on the inside sleeves of his jacket and shirt. Authorities also made out the barely legible lettering inside his boots: 'J W B - th.' "