In May of 1856 Senator Charles Sumner of Massachusetts was brutally assaulted on the floor of the Senate by Congressman Preston Brooks of South Carolina.
Brooks was the nephew of Sumner's fellow Senator, Andrew Butler, who Sumner had attacked by name in a great anti-slavery oration several days before. Sumner had mocked Butler's facial disfigurement following a stroke, but more importantly had called out Butler's beloved institution of slavery as being built upon the rape of Black women.
Then and now racists have tried to defend Brooks as a man of honour defending his family name, with Brooks becoming a celebrity throughout the south after beating Sumner in a surprise attack while the Senator was trapped behind a desk. Congressman Keitt, another South Carolinian, stood beside Brooks with a pistol and cane threatening people not to come to Brooks aid.
In our timeline the assault was an important milestone along the way to the American Civil War; not so much as a cause of tension but by making it clear just how far apart northern and southern political discourse was and how federal politics was breaking down as a shared civil space.
OTL the assault almost killed Sumner- he was taken out of the chamber a mass of blood, and his head wounds became infected.
But what if Sumner not only died, but had died during the actual attack? Say that Brooks isn't pulled away from Sumner for another ten seconds and actually cracks his skull- or that in the final stages of the attack as he held Sumner by the lapel with one hand and beat him with other he lets go and Sumner's head catches the corner of a desk?
What would be the consequences if an anti-slavery Senator was murdered on the floor of the Senate? How does this effect the 1856 Presidential elections? Can Brooks get away with a mere fine as per our timeline, or would his trial become a flashpoint for an earlier sectional crisis?
Side thought: What if Keitt, deliberately or accidentally, fires his pistol during the stand off and wounds or kills one of the bystanders?