SO yeah, he was an innocent victim. The fact is, he entered things into argument that could not be refuted with rhetoric, so the duller one had to attack.
He had no other tools.
The “duller one” could not refute with rheotric because he had suffered a stroke. While it is possible that Butler had personally insulted Sumner on a previous occasion (I’m somewhat skeptical about this fairly-recently “discovered” detail), berating a man incapable of speech for this and for his physical incapaciaty at the same time was (and is) in bad taste.
Fifty years earlier, or five hundred miles south, the result likely would have been a challenge to a duel. Brooks considered a duel, but after consultation with several other representatives concluded that that would not be appropriate, as Sumner was no gentleman. (Interestingly enough, Brooks had previously engaged in a duel with a man who would become a Senator from Texas in 1859).
Opting for caning instead of a duel was a well-deserved insult.
Of note, Douglas, while being insulted along with Butler by Sumner, observed “this damn fool is going to get himself killed by some other damn fool.”