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.

The OP's point was about morality, and it is as immoral as cheating in any system. Whether it's a crime is entirely different. Crime necessitates a judicial system, and the whole framework of crypto currencies has be outside the purview of any government or judicial systems. So the word crime itself doesn't mean anything in this context. Secondly, your argument relies heavily on the gravity of this act, which is entirely derived from the implied value of the coin. In isolation, this is no different from someone cheating in a computer game. Note that you cannot make the computer game analogy with other markets and banking systems easily because of dependence on other inputs. This, in contrast, is completely isolated from the rest of the world. As such, the guy may not do anything with the coins, and they would cease to exist (in their universe much like how they came into being). Would your argument still hold ? The fact that people have created a secondary market out of the coins, which came into existence out of thin air in the first place, doesn't automatically determine the criminality of the act.

This doesn't really address the morality issue. Suppose you could get away with it. The question remains, did you hurt people who didn't expect it etc.

When someone comes to play poker with chips, is it immoral to beat them and take their chips? No, they factored it in.

However, is it immoral to figure out a new collusion strategy and beat them?

Was it immoral to figure out a card counting strategy to beat casinos?

In other words, is it immoral to figure out new strategies in what is essentially an experimental game where participants assume there is risk?

In well established games like poker, which explicitly have gentlemen's agreements of "no collusion", it may be immoral.

But in a new experimental system such as a cryptocurrency whose software may be buggy, is taking advantage of the bugs really immoral?

When is it "buyer beware" and "a fool and his gold are soon parted"?

Isn't there any threshold for taking the money of a risktaker?

> You still run afoul of countless computer security laws

Only if you're citizen of/doing it in a country with such laws.

And with all that said, this type of activity is still attractive to some folk.

I'm content getting my kicks out of intentionally running the occasional red light.

But it takes all types. Imagine how boring the world would be without crime!