Yep, your snail is a Nassarius, although it's not one of the species usually implicated in killing live animals. They can sense the body fluids in the water, and are attracted to anything that might be food, even if it's not decomposing. A crab molting or a Stomatella dropping part of its foot probably "smells" the same way an animal smells when it's been ripped apart by some predator. Consequently, your snails go on the hunt, and will finish off any vulnerable animal they can reach. It's life on the reef, and an adaptation that is at least partly responsible for the success of the family Nassariidae...there are several hundred species in the family.
http://www.gastropods.com/9/Shell_3309.html
Sorry about your crab,
Cheers,
Don