Originally Posted by biotoxin
I find them out of their shells on the sandbed so i doubt its a molt.
That is exactly how a molt will appear. The only difference will be the anterior (butt) end of the crab will be missing if a molt and intact if a dead crab.
I could not find a pic of an actual molt but on the pic below you will notice what looks like a "tail" of sorts or abdomen. If that is missing, then it is most likely a molt as the crab shell will look identical in every other way including color...
If you are sure they are dead, then I can only conclude they where in poor health when you got them and/or acclimated too quickly.