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.