One of the great things about living in the San Francisco Bay Area is the close proximity to fresh seafood. Especially fresh Dungeness crab. We don't eat much crab but at the peak of the season when prices dip and live crab can be found for a really good price, we have to splurge.
After we buy them from the Asian grocery, we put them in the freezer to chill. It makes 'em "sleepy" so they're not so feisty before being rinsed and tossed into the steamer. I learned this trick from Alton Brown's show on lobsters, I think.
Trying to maneuver a live crab that is fully aware it is about to be cooked is an exciting operation. Once I tried killing one by poking a chopstick between its eyes. Didn't work. It grabbed the chopstick in both pincers and tried to pull it back out.
So now I freeze them for a few hours before they go into the pot. 18 minutes later, it's time to remove, rinse, and feast.
No butter necessary.