Well, these are wilder crab stories than I'm used to! Lol
I have no doubt that an emerald could and would kill a sick fish, or a sleeping one that was in reach, but most often unexplained fish deaths are not murder. My experience with the emeralds is that they attack sessile or slow moving animals with impunity. I'm not doubting anyone's stories, just to be clear. It just wasn't what I was expecting to hear!
As for the hermit question, the only substantial difference I know of between red and blue legged hermits is that red legged are natural predators of certain anemones such as condylactis.
Someone stop me! Please! But... not just yet...
55 Gal Tropical FW
55 Gal Mixed Reef