Why earthworms will be killed when salt is sprinkled to them?

If you sprinkle salt on a worm or snail, it creates a salt concentration gradient, wherein the fluid inside the worm is hypotonic with respect to the fluid on its surface. The water comes out of the worm's cells by osmosis until the solutions on either side of the worm's skin are isotonic. Worms don't like that.

The salt takes all the moisture from the snail/worm (what they use to live) and eventually kill them.