There are ways to address concerns about abuses of government power, he chose the nuclear route. Whether exposing the abuses of power that were happening is worth the side effects remains to be seen.
There are, but when you are likely to get brushed under the rug, other approaches need to be used. He essentially blew a hole through the rug, meaning there was no way to hide his message.
Was the way he did things the best way, it is hard to say, since I don't fully grasp the workings of the agency, but I suspect that there are too many people with vested interests in hiding their and the agencies failings? Sometimes in politics you need someone to put their neck on the line for the greater good, but it has to be done with care since otherwise to have collateral damage and possibly a miscommunicated message. IMHO Snowdon probably did something many people would have wanted to do, in the sense of causing change, but are too stuck in the political labyrinth to achieve anything. Don't underestimate the weight of government and bureaucracy to block real change. Too many stake holders who either have vested interests or don't want to experience change.
However you look at things, Snowdon was brave, but he did follow his convictions to the end. I think many of us would be too coward to do what he did.