They wanted Lincoln dead

To maintain their grip on the nation, Republicans couldn’t allow Lincoln to welcome the Confederate states back into the Union with their voting rights intact. The Republicans had Lincoln killed so they could keep control over this large voting bloc. The Secretary of War, Edwin Stanton, was the mastermind behind Lincoln’s assassination.
James William Boyd was Stanton’s assassin. When Boyd failed to show, Vice-president Andrew Johnson found John Wilkes Booth a willing replacement. Booth was able to take advantage of Stanton’s preparations, but he was not told and did not know of Stanton’s involvement.

• Knights of the Golden Circle: Secret Empire, Southern Secession, Civil War (Conflicting Worlds: New Dimensions of the American Civil War)
• The Escape and Suicide of John Wilkes Booth
• Rebel Gold: One Man's Quest to Find the Hidden Treasure of the
• Stanton: Lincoln's War Secretary
• John Wilkes Booth: By a Man Who Helped Him Escape (Annotated)
• The Templar Mission to Oak Island and Beyond: Search for Ancient Secrets: The Shocking Revelations of a 12th Century Manuscript
• The Reason Lincoln Had to Die
• The Curse of Oak Island: The Story of the World’s Longest Treasure Hunt
• Chasing Lincoln's Killer