Why was Abraham Lincoln assassinated?
His assassin, John Wilkes Booth, was a Southern sympathizer and he hoped that the death of Lincoln, VP Johnson and Secrertary of State Seward (all of whom were targeted to die the night Lincoln was killed) would throw the Union government into such turmoil that the South would be inspired to win the war. But this would have been wildly improbable after Lee's surrender 5 days earlier.
President Lincoln was killed because the conspirators, especially John Wilkes Booth, were angry at how the Civil War was fought and the fact that General Ulysses S. Grant stopped the prisoner-of-war exchange toward the end of the war. Booth was an ardent supporter of slavery and thought the South should have been left alone. Booth hated Lincoln and what he stood for, and was distraught by the destruction the war wreaked on the South. Before General Robert E. Lee surrendered, Booth had planned to kidnap Lincoln, hoping he could be exchanged for Confederate prisoners or even end the conflict.