If they did this, the attack would cause a panic, and Bitcoin would likely instantly loose its value, thereby undermining the validity of their own (illegitimately acquired) wealth. Not very smart...
The inventor of Bitcoin once described this scenario, saying it would be smarter and more profitable to simply mine bitcoins legitimately...
"Illegitimate" is the wrong word. I meant wealth acquired without following the proper rules (ie. mining on the longest chain, because the attacker would purposefully try to fork the chain from a past block.)
What if they simultaneously transfer a huge amount of BTC to multiple exchanges (double-spending), exchange to USD everywhere, running away with a bunch of dollars, while the rest of the bitcoin world slowly realizes they've been had?
How would the attacker obtain the "huge amount of BTC" in the first place? If he buys it, sells it, then executes his double spend and Bitcoin loses its value, the attacker would have made no profit.
A double spend attack only allows an attacker to double his BTC before the block chain has to be forked, and before the whole community notices the attack. You can't send the same BTC to multiple exchanges. You have to execute one double spend attack for each exchange. But the community would detect the attack after the first one.
(Double spend attacks would be noticed when multiple blocks in a row are replaced or, in Bitcoin's terminology, when a reorganization occurs: http://blockexplorer.com/q/reorglog)
The inventor of Bitcoin once described this scenario, saying it would be smarter and more profitable to simply mine bitcoins legitimately...