The gore in these novels can enhance the horror aspect. For example, in 'Another', the gory deaths create a sense of danger and mystery. It makes the readers constantly on edge, wondering who will be the next victim.
Hard to say. Sometimes movies inspired by real events take liberties for dramatic effect. Maybe do some research on the filmmakers' intentions or look for interviews.
It's subjective. Different people have different opinions on what constitutes the best gore novel. It often depends on personal preferences for storylines, writing style, and level of gore.