The Good Guy by Dean Koontz
Timothy Carrier is an ordinary guy who enjoys a beer after work. But tonight is no ordinary night. Instead, Tim will face a terrifying decision: Help or run. For the jittery stranger sitting beside him at the bar has mistaken Tim for someone else--and passes him a manila envelope stuffed with cash and the photo of a pretty woman. "Ten thousand. The rest when she's gone."
Now everything Tim thinks he knows--even about himself--will be challenged. For Tim Carrier is the one man who can save an innocent life and stop a killer as relentless as evil incarnate. But first he must discover resources within himself that will transform his idea of who he is and what it takes to be the good guy.