It turns out we've been building massive surveillance engines and stuffing them with phone data, social media conversations, search terms, and who knows what else. And it turns out that we're so afraid of terrorists that we think this is a good idea, even though the 9/11 attack killed less than a quarter of the number of homicides in any year from 2000 to 2010. And maybe it's even stopped an attack or two, in which case it's saved some lives.
If saving lives is the criterion for whether we should do this, however, I wonder why we'd stop there. Why wouldn't we use the same approach domestically? If we found people whose profiles suggested future murders, why wouldn't we go through their lives and contacts and find ways to get them off the streets? Wouldn't that be the responsible, prudent thing to do?