he was a democrat who used to own slaves, picked by lincoln to pivot to the center and try to win some southern votes.

It didn't work, obviously; the South saw the election of Lincoln as an act of anti-slavery aggression and seceded. So we get the Civil War, Lincoln's strong leadership, an eventual victory by the North, a Reconstruction plan is hammered out including things like reparations to all newly freed slaves—

And then Lincoln is assassinated, and we get an ex–slave owner in charge of everything. There were various power struggles between him and the Radical Republicans. The end result is that the Reconstruction was nowhere near as successful as it could have been (e.g., Johnson axed the reparations plan).