I have a yellow one. Had it for 2 years without any problems. Thought the same thing and then it recently ate my leopard Wrasse lol I'll never trust these guys again!
It is opportunistic. So if the fish is weak or on its way out then even a hermit crab will eat fish. It doesn't actively hunt. It's good in the sump to eat Detritus or in a large tank where there is plenty of space for a weak fish to hide