Blue Knuckle vs. Fire Shrimp...???

[doch]

So, earlier today I picked up 3 fire shrimp from an LFS... later in the evening I notice that one of the is not moving, I inspect closer and there are 2 hermits (one blue knuckle, and one tiny blue leg) eating him. Are blue knuckles known to kill shrimp and eat them, or is it more likely that the shrimp just died, and the crab took advantage? BTW, the other 2 fire shrimp seem to be doing fine.