Security people don't necessarily think about the past, because it isn't relevant. The death/injury ticker isn't really relevant. The assassination of Abraham Lincoln and post-WW1 anarchist bombings are not related and aren't relevant to today.

Security people do think about what motivated individuals can do. If some lunatic with a semiautomatic rifle can massacre dozens of people in a school or movie theater or shut down a major city for days, what could a trained group of fanatics do? There's a nearly limitless number of horrible things that can happen, and many of these things are real risks, just waiting for the right fanatic or lunatic to go for it.

The open-ended nature of this anti-terrorist campaign/effort/whatever reflects the open-ended nature of the risk.

That said, I think Snowden did a valuable thing. I think the approach the government has taken is too secretive and too much of an overreach. In an attempt to foil terrorists, we're undermining our democracy and the integrity of our institutions.