What are your water parameters? I would always check them fisrt before you start putting blame on tankmates. I really doubt a hermit could catch a fish. My friend kept a red-leg hermit about the size of a softball in a tank with a cardinal, a scooter blenny, and a blue damsel without problems. So, I really doubt it was the hermit. Hermits are oppertunistic feeders which would eat a dead fish if it had the chance. Hope you find out whats killing your fish.