According to the misappropriation theory of inside trading, it seems you would be doing something illegal. Or perhaps you could be convicted of acting as an accessory to securities fraud. Whether it's moral or in keeping with the spirit of capitalism I guess depends on your interpretation of those two things. Personally I don't think capitalism has a finders-keepers rule when participants buy stolen goods, but you may read it differently.
FYI, the information I'm basing this on comes from a similar case in the Wikipedia article:
"""
In 1997 the U.S. Supreme Court adopted the misappropriation theory of insider trading in United States v. O'Hagan, 521 U.S. 642, 655 (1997). O'Hagan was a partner in a law firm representing Grand Metropolitan, while it was considering a tender offer for Pillsbury Co. O'Hagan used this inside information by buying call options on Pillsbury stock, resulting in profits of over $4 million. O'Hagan claimed that neither he nor his firm owed a fiduciary duty to Pillsbury, so that he did not commit fraud by purchasing Pillsbury options.[16]
The Court rejected O'Hagan's arguments and upheld his conviction.
The "misappropriation theory" holds that a person commits fraud "in connection with" a securities transaction, and thereby violates 10(b) and Rule 10b-5, when he misappropriates confidential information for securities trading purposes, in breach of a duty owed to the source of the information. Under this theory, a fiduciary's undisclosed, self-serving use of a principal's information to purchase or sell securities, in breach of a duty of loyalty and confidentiality, defrauds the principal of the exclusive use of the information. In lieu of premising liability on a fiduciary relationship between company insider and purchaser or seller of the company's stock, the misappropriation theory premises liability on a fiduciary-turned-trader's deception of those who entrusted him with access to confidential information.
"""
Of course, IANAL and I could be wrong, but it sounds like the same sort of thing. Information is created about a company by a third party, and someone else uses that information to make trades.
Then again, these two situations are also not analogous, because this guy's trades on MtGox were based on public information (namely that the value of bitcoins was falling precipitously). So he might not have done anything technically illegal, were this a real exchange. However, his trades would probably have been rolled back in a real exchange (assuming the trades were enough to move the market like this), so that still supports my overall perspective that he should not expect to get away with the money.