Charles Sumner was actually correct; Brooks was offended by the truth. Modern day leftists suffer the same defect. McTurtle deserves the caning, Sumner did not.
Lincoln was too moderate for Charles Sumner and the vindictive Radical Republicans, a faction that would have hanged large numbers of defeated Confederates. He voted to impeach Andrew Johnson. He wanted to dissolve the Southern states, strip its citizens of the right to vote and have the states ruled as conquered territories. He finally met his match when he accused President Grant of being a corrupt tyrant.
Sumner would have made a great Bolshevik. Like Thaddeus Stephens and the other Radical Republicans, Sumner was one of the premier hate-mongers of his time. It should come as no surprise that they were admired by Marx and Engels who closely followed the American Civil War.