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)
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)