Just about any crab is a potential threat to other invertebrates. I have my doubts with the emerald crab killing it and most likely it was already succumbing (starvation perhaps?): Shall we say, wrong place at wrong time for the emerald. I have; however, seen a variety of hermit crabs (even the "peaceful" scarlet reds) rip/chew a hole through the sheath in order to pick at the actual animal on top of picking at healthy snails, corals, serpent stars, etc. Misfits I tell you!