Brooks didn't challenge Sumner to a duel because Brooks was a coward and was scared to death of the unpredictability inherent to a duel.
So he waited until he had two strong friends with him, armed himself with a club, and waited until his unarmed opponent was sitting down at a disadvantage. Then he clubbed him thirty times.
"Got off light"? You believe that the proper response when someone says that someone else is pro-slavery (and is right in saying so) is a permanent brain injury? Who raised you?
Sumner, however, did not keep his mouth shut as you falsely claim. He rehabilitated himself and returned to publc life. He continued to oppose the buying and selling of human beings, and lived to see chattel slavery extinguished in the entire country.
Brooks, the sucker-punching loser, manfully died of a cold before he was able to "honorably" run like a rabbit from the Union Army. He made the titular princess of "The Princess and the Pea" look tough.
You are a real life purebred yankee liberal, bless your heart.