From the picture on that site, it looks more like a scarlet reef hermit. That one isn't supposed to be a problem with eating fish.
hmmm. There are many different reef hermits, the one I'm talking about is called the White Spotted or Shell-Breaking reef hermit. I attached 2 pictures.
They're reddish, with white spots, and hairy legs. But the main thing is that they are bigger than most hermits, getting to be 5 or 6 inches.
Fish stores get so many different kinds of hermits, in all sizes and all colors, it must be impossible to identify them all.
I can only tell you what I would do in your situation. If a healthy fish was attacked and eaten in front of me by a big hermit crab, no matter what kind, I'd take the crab back.
If you keep him, don't give him any new larger shells to move in to. The larger the shell, the larger the crab will get. :-}