Question: Reverend Lowe says that he's been killing people because of the sins they committed but why did he kill Brady? He never committed a sin.
Question: Why did Marty insist on shooting the firework at the werewolf? It took him far less time to turn the bike round and roar away than it did to light the rocket it and aim it.
Answer: Simple fight or flight response. His first thought is to fight whatever is stalking him. Once he discovers it's an actual werewolf he takes off.
Answer: He might have been hoping that the sudden loud bang and/or flame would scare the beast (werewolf) away - most (if not all) wild animals respond to noise and are afraid of fire.
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.