The scene in the movie where Rorschach deals with the child killer, has him butcher the guy like Jason Vorhees.
In the book Rorschach is much more restrained, he handcuffs the child killer to a furnace, and gives him the option to saw off his own hand. His action/inaction speaks volumes, the change in plot/character you might think is minor, but imo its major; a bastardization of the character.
So too was leaving out Rorscharch's genesis; the scene in the book where he describes in detail the murder of Kitty Genovese, the subsequent apathy of the witnesses, and his appropriation of her Manhattan fabric dress that he goes onto to use as his mask, is essential to setting up his character, Snyder chose to omit these crucial details, just one of dozens of poor choices made.
By cutting the sub-plot of the island, which leads to the denouement of the book, is just another case of why the film is mediocre, great attention detail lavished on some scenes, other scenes are hamstrung and bastardized by a lazy screenplay and poor choices.
There's little point in being faithful to some sections of the book, while being unfaithful to the most essential pieces.
