You can make a reasonable argument for why Edward Snowden was wrong to release the information he did to The Guardian and The Washington Post (for instance, here's Jeffrey Toobin and here's Josh Marshall making that argument). But if you're going to turn Snowden into a villain, you'd have to show that the leaks did some kind of demonstrable harm to American national security. Even if you don't find Snowden's action heroic, it's quite possible that leaking this classified information was illegal and wrong, but nevertheless didn't do much damage or make us less safe than we otherwise were.
So what is that harm? What the government has been saying so far isn't all that persuasive. While some of the details are new, we've known for years that the NSA was tracking phone and Internet traffic. This reminds me a little bit of 2006, when The New York Times reported on a government program to track and disrupt financing for terrorist groups through the banking system. The Bush administration and its conservative allies went nuts, some arguing that the journalists who wrote about it should be prosecuted, and claiming that now the terrorists would know we were going after their money. But not two weeks after the September 11 attacks, President Bush publicly bragged about how "We have established a foreign terrorist asset tracking center at the Department of the Treasury to identify and investigate the financial infrastructure of the international terrorist networks," and how the United States would be working with its allies to disrupt that financing.
In the same way, intelligence officials are now saying that the terrorists had no idea that we might be tracking their communications. But that just doesn't pass the smell test.
And if any of them were that ignorant, you could even argue that the revelations could have beneficial effects. For instance, let's say you have two terrorists, Mitch and Steve. Because (like most terrorists) they're idiots, they didn't realize until now that the United States government might be tracking their phone calls and emails, and they've been blithely communicating with each other about their plans to take down the American hegemon. Had these programs not been leaked, they would have gone on doing so. Yet that doesn't necessarily mean they would have been caught before. There is a chance that the NSA would have located them, but there's also a chance they would have been missed amid all those billions of calls and emails. How big is that chance? I have no idea, and I don't know if anybody else does either. Maybe 90 percent, or 1 percent, or .001 percent. But it's bigger than zero.
Now that they've learned that their communications are not secure, Mitch and Steve have to invest a lot more time and energy into communicating with each other. Maybe they're only passing messages by trusted friends who happen to be traveling between where one is and where the other is. Maybe they're using carrier pigeons. In any case, if they're going to go from using phones and email to reverting to pre-21st-century communication technology, it will make efficient operations more difficult, further reducing the already small chance that they will one day be able to carry out a successful attack (remember, like most terrorists, they're idiots). As an added bonus, they may become more paranoid about the U.S. watching them, leading to more intra-terror-cell suspicion and infighting.
Or maybe it won't make much difference to them; I'm just speculating here. But so far, the intelligence community hasn't offered any particularly persuasive evidence that the leaks have damaged national security. They're just speculating, too. And their speculation doesn't seem to have much more evidence behind it.