Should we bioengineer a new species of crab, that is 150 times the size of a regular crab ?

If some get loose, how many people will they crush and eat?

Only if they look like this. 

Yes, as weapons.

Think about giant hard shelled creatures with huge claws that eat meat. We'd wipe out all our enemies with them.

Possibly. I understand that there is a theoretical limit to invertebrates so that would be an interesting experiment.

Depends is this new super crab a weapon or just a tasty source of food?

no, that would be too big