There's a ton of laws and guidelines that you have to abide to whenever you're making any sort of feature-length film. One of the biggest no-no's that filmmakers tend to generally avoid is the killing of animals. Who wants to watch a movie where at one point an animal actually dies?
Apparently the filmmakers behind the independent thriller 'First Winter' could care less about people's viewpoints on real animal deaths on screen. Why? Because they ended up killing two deer while filming their indie. Not a smart move, especially since they're getting all sorts of hell over that decision.