To me, this radical change in stance suggests that there is in fact a credible domestic threat against the United States - that Obama, on assuming the presidency, learned some things that made a deep-enough impression on him to transform his stance on national security. I don't like that conclusion, but to me, that's the simplest interpretation of his behavior. (I'd love to hear other, less frightening interpretations. As it is, I try not to think about this stuff too much.)
If so, perhaps many of the actions of this and the past administration are justifiable as lesser evils - perhaps. However, the executive branch shouldn't get to make this determination alone. It's not necessarily the administration's decisions that are the problem here, but the complete veil of secrecy around them that's keeping us from having an informed debate.