It's worth mentioning that the beating seemed to be mainly over a personal insult contained in the speech -- the Southerner's brother having recently been convicted of a crime or something to that effect being mentioned. I do strongly suspect, however, that the Southerner's willingness to resort to brutal violence over what is ultimately a petty jab might have been informed by being a slaver.
The senator from South Carolina has read many books of chivalry, and believes himself a chivalrous knight, with sentiments of honor and courage. Of course he has chosen a mistress to whom he has made his vows, and who, though ugly to others, is always lovely to him; though polluted in the sight of the world, is chaste in his sight;—I mean the harlot Slavery.
If we're clarifying this in babysteps, the person referred to here is then-Senator Andrew Butler, the first cousin once removed of Brooks, the actual assailant, who decided days ahead of time not to challenge Sumner to a duel but instead blindside him as he did. It was a gold-headed cane, making it also a much heavier blow than if it was just a wooden one, and it was used deliberately because Brooks switched to using the piece with the gold head once the cane broke in order to keep striking.
In the most minor defense of the other northerners, there were two co-conspirators (southern Representatives) whose sole job besides clearing the women out beforehand seemed to be preventing others from intervening, including one threatening others with a pistol!
It should surprise no one that it was two Representatives from New York who ultimately stopped the fight, likely saving Sumner's life. Though both were themselves obviously capitalist scum, it makes sense that they wouldn't have the same stomach for direct brutality, or perhaps they just saw Sumner as a valuable asset -- which, to be fair, he certainly would become after this event.
Massachusetts Representative Anson Burlingame publicly humiliated Brooks by goading him into challenging Burlingame to a duel, only to set conditions designed to intimidate Brooks into backing down. (As the challenged party, Burlingame, who was a crack shot, had the choice of weapons and dueling ground. He selected rifles on the Canada side of Niagara Falls, where U.S. anti-dueling laws would not apply. Brooks withdrew his challenge, claiming that he did not want to expose himself to the risk of violence by traveling through Northern states to get to Niagara Falls.)
Yea everybody always talks about how cool it is when Senators fought on the floor but Sumner was an abolitionist who gave a speech about the evils of slavery and then that douche was mad so the next day he came up behind him and beat him with a cane while he couldn't get up from his desk.