Why was lincolns death important?

It extinguished the mood of reconciliation which followed the news of Appomattox, and caused a wave of bitterness in the North against the South in general and the Confederate leadership in particular, who were assumed, incorrectly but understandably, to be behind it. An ironic beneficiary of this was Jefferson Davis, whose resulting imprisonment in Fortress Monroe turned him into a "martyr" and put him above criticism even for those Southerners who regarded him as an incompetent leader. It also transferred the Presidency to Andrew Johnson, a maladroit politician who soon got into bitter quarrels with Congress, which in turn led to the imposition of military rule in the South, and a foredoomed attempt to rebuild Southern society as a colour-blind democracy. The results were as bloody a failure as might have been predicted.