pathfinderq1
Explorer
Yeah, it looks like this calls for a rogue (if you want to stay with the crossbow idea), or an invoker (wielding a rod or staff implement reskinned as an elephant gun- or perhaps a Boomstick...) 

Not all character concepts are going to work with D&D. Maybe loosen up on the concept some.
If you find your concept and class in conflict, before condemning the entire system, try another class.
Probably the same sort of person who can manage to deny their existence in the first place. A snark-hunter. You'd have to be a bit of an odd duck, but we are talking about zombie Theodore Roosevelt here.Atheism is a lack in belief of god. Atheist literally translates to "One without belief in god." An atheist god hunter doesn't work as a serious concept because, well, what kind of person dedicates their life to hunting something they don't even think exists?
In any case, you could believe that the "gods" -- as most common folk in your campaign call them -- are all pretenders, and not worthy of the adoration they are given. You could accept their power without granting it your worship. Curiously, your character would be placing themself as a higher moral authority than the gods.
[Side note: in order to get along with Clerics/Paladins in the group, the character would work with them until they led him to their deity. Then he would kill both the deity and the follower. A very gentleman like agreement....]