I've read most (all of them? Except Feersum Endjinn, it was just too hard to read to enjoy it - but that's me) and I do think Banks has a lot of violent and sometimes gruesome parts. The Archimandrite Luseferous springs to mind (not part of the culture series, more an anti-culture book, it feels like the mirror universe in Star Trek, E.g. in DS9, but it's so culture-like, I include it).
Yes. Interesting question. I’d say that in terms of the goriness of the writing, the island is particularly bad.
Thematically though, in terms of the series overall, violence, oppression, genocide, torture, are all fairly common, although not as gruesomely depicted for the most part.