Jane Coslaw: Aww, look out world, Marty the Great didn't get something he wanted.
Uncle Red: Where's your Mom?
Marty Coslaw: She and Dad are out back lighting the barbeque, yeah, and Jane's walking around in all these new clothes showing off her tits, acting like no-one ever had tits before her.
Reverend Lowe: You see, you meddling little shit.
Marty Coslaw: You know who used to have a baseball bat like that? Mr. Knopfler.
Uncle Red: So?
Jane Coslaw: It looked like Bigfoot had used it for a toothpick.
Milt Sturmfuller: Ow! That's it. Oh, that hurts my parts.
Uncle Red: I mean, uh, what the heck you gonna shoot a .44 bullet at anyway... made out of silver?
Mac: How about a werewolf?
Uncle Red: You wanna know what I think?
Marty Coslaw: No, we just called you out here to admire your pretty little face.
Uncle Red: You better watch your mouth, right now. You're on thin ice with me, son.
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.