By all means keep the case. Next time you build a computer i'm sure you don't want to slap down money for another one.
I'm using a computer case from 2000 still that my dad first used, then a friend, then another friend. It made it's way to me, i ripped out the innards and tossed the latest hardware in there and here i am.
But, if the case is old AT style (you didn't mention age of the computer), then by all means destroy it.
If the case is proprietary, destroy it.
If the case is a non proprietary, non AT, and is a cheapo, then contemplate destroying (although i'd keep the case, whatever saves $20-$70 dollars on a new home built computer).
I recommend combustion potato gun with a frozen 20oz. diet coke. A potato gun is not a fire arm. A combustion variant does utilize hair spray or car starting fluid for propelling the projectile. If you really don't like fire, then use a pneumatic potato gun. A frozen 20oz. diet coke projectile you'll be very satisfied with blasting at the computer more than any melee weapon i can promise.
Other than that 20 pound sledge (it's put big holes in cars and is very satisfying).