I believe it is a crime, both in spirit and on paper. In spirit, you took something that was not yours and not meant for you, and by selling the coins you decreased the value of everyone else's. On paper, let's say that the developers behind whatever coin you hack decide that they are going to take no action. You still run afoul of countless computer security laws designed, however poorly, to protect people from hacking, so at least in the first world you could find yourself in a legal bind if your identity is ever discovered. Even if the devs don't go after you, the rest of the community may, or the government may of its own accord. Now, in keeping your identity private, you have to launder the money, putting you in violation of even more laws and ones that the government will most definitely care about. The biggest player left is probably the IRS, who will take all kinds of interest in either your newfound, undisclosed wealth, or your half million dollars in untraceable gambling wins. It's an extremely reckless move unless you are effectively a pirate living in some lawless part of the world with good connections to foreign banks.