The one problem I have with it. Heavy spoilers!
I usually don't care about plot holes. All movies have them if you think hard enough. That's trivial, at best.
What bothers me much more are bad characterizations to build plot. It's simply bad writing and bad planning. In this instance, every serial killer has an MO. They like killing this way or that. They choose victims that look like this or that. They don't really deviate from their ways.
Here, we have a killer who kills only the guilty. To date, he's killed quite a few people and all of them have been parolees. He didn't go after their families. You see where I'm going with this.
Clinton Davis' decision to go after an innocent woman and child is preposterous! Now it can be said that killers will make MO changes when pushed into corners. That is true and Clinton was pushed, but not to the point that he'll kill someone innocent. His entire reason for killing was revenge and this woman and child had nothing to do with any of that. No.
It would have been more likely that he'd kill the one who wasn't innocent. The proven liar who broke the law: Mitch. I really think it was a bad decision to make him try and kill Mitch's family. It is very unlikely Clinton would really do this. It was to serve a plot point and wasn't well thought through.
I will go ahead and address the issue about why Mitch wasn't in prison for murder. He wasn't the killer, Clinton was. The medical examiner said the death blow was blunt force trauma to the head from a hammer. Even if Mitch had run him over, he was going to die. The trauma to the head occurred before Mitch ever crossed his path. He hit a dead man. The most he could have been charged for was a hit and run, reckless endangerment and maybe some form of assault due to the car hitting him while he was alive.
He would have been charged and would probably not get jail time since it was implied that this was his first offense. He would however lose his job. Still, being a lawyer he could do very well for himself.
Random Thoughts: http://goo.gl/eXk3O