It's more inferred than it is actual. Pick out the details that are going on.
They are locked away for six months. Proporo has provided all sorts of entertainment: dancers buffoons (court jesters or fools), beauty. All this wealth being spent on the guests.
When they came the guests, bolted and welded the gates, they were there for the long haul. So they wanted to isolate themselves from the Red Death.
Now all we have to do is find something that says they were indifferent to those not there. It took 1/2 an hour to die. And yet the prince was happy, dauntless [fearless and determined] and sagacious [ his mind was all there].
That last sentence is the one I'd pick after going through all the details I've listed. He couldn't care less. The sentence before is a good answer as well. The scarlet stains didn't take but 1/2 an hour and they were so hideous they did not evoke sympathy. It's open ended. You can just about pick anything if you put in enough detail before it, if you can.
