Shrimp and crab shed their "skin" periodically, called molting. After a shrimp molts, it's new skin/exoskeleton is kinda soft so it will hide for a day or two while it hardens up. Yours could just be in hiding and the crabs are just cleaning up the old molted skin. Even though hermits will eat anything they can get their hands on, I can't fathom one catching - let alone killing - a shrimp. If what you're seeing isn't a molt, then most likely it died without any help from the hermits.