Factual error: The story takes place during spring of 1976 but Marty's parents drive a 1979 station wagon.
Factual error: When Marty is on the covered bridge, after running from the pastor, he hears a tractor running. He starts yelling for Mr. Zimmerman, who is driving the tractor, and Mr. Zimmerman stops the tractor and walks to the covered bridge. It would have been nearly impossible for Mr. Zimmerman to hear Marty yelling for him over the noise of the tractor.
Answer: Firstly, Reverend Lowe is deluding himself by saying he only kills people because of their sins. He is in fact wracked with guilt over his actions and has at least one vivid nightmare about this. Secondly, in the Christian faith all humans are considered sinners so this gives Reverend Lowe a justification for his beliefs. Anyone he kills would be a sinner in his eyes, including Brady.
BaconIsMyBFF
I think the first part is more accurate. It really has nothing to do with the idea of original sin or everyone being a sinner since he states he only punishes the evil, corrupt, and immoral. He tells Marty he'd never hurt an innocent child. He just really has no control of his Wolf side and is lying to himself about why he kills.