I know its funny to thimk about, but you should have seen him swimming around with crab legs hanging out. Don't want to think he killed it but thats the 3rd crab I found out of shell dead. I witnessed this one in his mouth. Maybe he just picked it up off the bottom.
My Diamond doesn't kill my cleanup crew but my blue spotted watchmen goby kills all my snails. He literally carrys the shell around in his mouth and stacks them behind his favorite cave. I have to clear his area out after a couple weeks its annoying haha.