Shots Fired: The Assassination Of Jefferson Davis

  It would probably depend on the exact circumstances.  I.e. what date exactly and the motives of the assassin.  If their a supporter of the north and opponent of succession it would probably harden the desire to fight to for seccession.  If someone who has a particular bug with Davis then the impact is likely to be different.  Being hit by some fire-eater who finds him too moderate would also have probable influence.

  From his Wiki entry Stephens was a distinctly reluctant rebel so he might seek some peace agreement with the north, although especially if after Gettsburg, this might not be welcomed in Washington.  He also repeatedly criticised Davis on things like the latters imposition of conscription and suspension of habeas corpus amongst other things.  As such you might see an earlier end to the war one way or another, unless he proved a significantly better leader than Davis.  Not sure what he would be like in selecting generals and commanders and how well or not he would deal with political infighting between military figures and similarly between assorted regional politicial figures.

  I suppose as an outside possibility Stephens, if rebuffed by Lincoln might ask Britain to offer its mediation to end the conflict, possibly with some sort of reunion but protection of rights for slavery in the rebel states.  Suspect Lincoln might well reject that as well.  Still unlikely to bring war between Britain and the union but it might bring more international sympathy for the south and increase internal hostility to the war in the north.  Which might or might not make a significant difference depending on how quickly things happen.  Still very likely to see the south defeated in such a scenario but it might take a bit longer and be a bit bloodier.