The government is running a top-secret program to create fighting machines. These people are bred to kill, and they don't even feel pain. You have no idea who they are. It's a government conspiracy.
Wait a minute...this plot's been done before.
But who's complaining? Didn't we all love watching Jason Bourne struggle to find out who he was after Treadstone took over his identity?
Besides, it's such an easy plot device. No one can prove it or disprove it. Of course the government says it's not real. They also say that Roswell didn't happen.
But it's an old plot device. Conspiracies in high places, Interpol agents sold to the highest bidder, FBI and CIA working for the other side. I'll admit, I used it too. What good is a thriller if all the guys that appear to be good are actually good?
That's not how it is in real life, though. The good guys are good, and our government would never hire assassins. That only happens in fiction.
At least, you can keep telling yourself that.