Violence even in horror movies has to have a logical placement in order to further the movie and keep continuity.

The point of slashers is to shock you but there still has to be content to make it believable even if you are using the supernatural. Look at 13 Ghosts for example, the ghosts were bound by metaphysical law which was the glass containment system that was built and engraved with the writings of ancient bonds. When they were released people did not just suddenly explode nor could they attack the humans through the containment cells they were in. Once freed they attacked with what would be concluded as logical choice of instruments. The nail guy attacked with a spike and hammer, the drowning woman attacked with her hands (claws), and it was logical because it was how these people died or what they had with them at their disposal as well as the set boundaries...it was consistent and it had substance; which by the way is rare for a mainstream horror movie with star actors to actually do.

So what I am saying is that if you start movie and give no background, then you enter a guy with a chainsaw to clear out a room of high school kids attending prom, you are left with a gaping hole and nothing but attempted shock value with no buildup to the shock.