Crab killed his tank mate possibly for shell

[Tarn]

Hello everyone,

I am absolutely devastated and in need of some advice.
My Crabs have always gotten along. my crab named Big, was actually the biggest in the tank but super gentle and social and I loved her so much (they are Australian crabs so our version of big is not like American version). Comet was the next biggest.
Comet has killed Big. I am not sure why this would have happened, Comet was acting like it was for Bigs shell but there were 4 appropriate shells for comet in the shell shop.
I don't know what to do, I have put comet in an isolation tank with the spare shells (not Bigs shell, I am keeping that) she keeps swaping into the shells and then back into her original shell.
I am scared to put her back in the tank incase she attacks someone else. Does anyone have any advice? I have no idea if it is safe to put her back?

Thanks
Tarn

[wodesorel]

I'm sorry for your loss.

What exactly did you see? It can happen, but with other shells being provided it's extremely rare for them to try to rip another crab out forcefully over only a shell. I've personally never had it happen once, but I have had brutal attacks for other reasons.

[DaTrininator]

Aw... Sorry for your lose .

[Tarn]

I did not see the attack, I got back to see Comet holding onto Bigs shell and refusing to let go. Bigs body was buried nearby. I think that she was pretty mangled as I saw two of her legs in the substrate. My partner found her and buried her so that I did not have to see and remember her that way. Comet is still in isolation, I have given her even more shells to choose from and she keeps swapping between them. I don't want to keep her by herself as I think it is cruel. How do I know if it is safe to put her back in with the others?
I have had them out of the tank for a few minutes together so that they don't forget each other.
Is there a way that I can tell if it is safe to put her back? She is now the biggest of the crabs.
Thanks for your help