A sin is –theoretically– any choice that displeases God. If it is unavoidable, then it is not a sin because there is no choice. For example an accident you caused is not a sin (unless you were imprudent). The problem is that we never know whether something pleases God or not. So we can only make assumptions. We assume that God loves every creature and therefore never likes when any living creature gets harmed. So we can assume as a rule of thumb for recognizing a sin that it harms somebody. If your choice causes harm to a creature, even a pet, then it is a sin.
We suggest the following definition:
A sin is a choice that causes harm to somebody.
Keith Giles formulates well why many people turn away from church. He writes (source): “It’s quite disturbing to me that so many Christian churches today are continually fixated on how sinful we all are. They constantly remind us that we are unworthy and that our sins are filthy and that this keeps us separated from God.” In another blog post (Seven reasons why Jesus was not sacrificed for your sins) he goes further and writes that “One of the lynchpins of Penal Substitutionary Atonement Theory [PSA] is that Jesus had to die on the cross to fulfill God’s requirement for a worthy sacrifice that could atone the sins of mankind once and for all”. In a theological fight against the idea of atonement he gives a list of Bible verses that turn it ad absurdum. I personally think that Keith is getting a bit too zealous there. Which is an example of how the Bible can be interpreted in very controversial ways. Theology is an eternal journey in boggy lands!