No, I wouldn't consider it rude. You gave it a try, and found the violence overwhelming, so stopped reading. I'm sure you can still participate in a discussion about the book, and you might find the other's thought about it provoking.
I belong to a book group going into 15 years now, and several times we have someone who couldn't finish a book. Their reasons became an active part of the discussion. Also, the person who suggested the book should take no offense, since we all have different taste!
One of our funniest memories as a group was the weekend we planned away at a fairly remote cabin resort. We had a lovely time, and on the second night which we designated for book group, we found that none of us had finished the book! Tar Baby by Toni Morrison - it was very highly rated, but none of us could get through it. We left all 7 copies on the bookshelf in the cabin. Hopefully future vacationers found it more inviting.