Ok, so we didn't prevent the maximum amount of wrongdoing? No program is perfect. Those pedophiles would've been just as free had they not developed the programs in the first place.
You're comparing reality against a theoretical world where we can both prevent crimes and never need to develop the tools necessary to do so.
As to the second point, sure, we don't know. That's tangential to my point. Let's assume the government is abusing their power. What does that have to do with these tools? Nothing, besides it maybe makes it a little bit easier for them. But, like I said, they'd do it anyways with sufficient motivation.
I still optimistically believe that an important tenet of liberalism is that you put some trust in your government, but recently it seems for many liberals that trust only extends to cushy (not to say I don't want them) domestic programs like healthcare, equality programs, or whatever the program du jour is. I'm willing to trust that our intelligence agencies aren't some nightmare Orwellian monster hellbent on subjugating the populace. Abuse of power could happen just as easily through other agencies, it just doesn't sounds as scary as when you slap a three letter acronym on it.