You can kind of describe things in print that wouldn't pass visual censorship, for example Mass Effect is very tame on violence, there's a lot of deaths but it's rarely going past cartoonish disintegration or ragdolling. In the books Drew Karpyshin describes Cerberus maiming a quarian in detail, down to what happens when the eyelids get torn off... and red sand withdrawal. I still have nightmares. Even a regular rifle volley has detailed biological description of all the gory parts that get shattered when someone's shoulder turned into paste. In games, you just see flashes on the screen.
Otherwise, it depends, very few adaptations are exact, in Lord of the Rings you have entire chapters and major secondary characters cut for pacing, but then battle scenes expanded instead of fade-outs in the book. It's just right. Hobbit is darker in movie form, definitely, original is a children's story in tone. A lot of people disliked the changes, but I loved it because I knew they won't give more high budgets for fantasy combat scenes on big screen any time soon, so I just wanted to watch huge formations of elves fighting orks and more Legolas. I don't care if it's not accurate, it was fanservice to me.
Bond has a curious history of adaptations. Goldfinger is much better on action as a movie, but removes the absolutely insane internal monologues Bond has about lesbians and Koreans in the book. Most 007 movies also doubled the sex compared to books. Roger Moore era films got more comical, and Dalton got gritty detailed scenes despite not actually adapting books, i.e. License to Kill borrowed a graphic shark attack straight from Live and Let Die, just changed the victims and context... Dalton movies are very close to earlier books in tone, still Casino Royale changed a lot of events because it needed to adapt the story where there's basically zero action in and is set during early Cold War... they kept intact the ball torture scene almost exactly tho.
Philip K. Dick adaptations usually change the story a lot, Total Recall expanded a 40-50 minute novella into a full length movie and added a lot of violence, but somehow simplified We Can Remember it For You Wholesale's story, and got rid of extra plot twists at the end. They also plugged in the three-boobed lady from the Golden Man, an unrelated short... there was a one season series Electric Dreams based on his works, also mostly inaccurate... if you watch one episode, make it "Kill All Others", it's very relevant.
I can't really think of THAT many movie adaptations that were significantly darker in print. Maybe with exception of manga, which is almost always more unhinged compared to anime... Even when it goes the other way around, the manga novelization (mangaization?) of Fate Zero is significantly more brutal than the original series. Which is also pretty dark compared to VN, oh the anime version of visual novel removed the sex scenes, that happens a lot unless they're adapting a nukige like Bible Black.