The death of a beloved animal and/or pet is one of those subjects that's very tricky to pull off effectively. Similar to other grim subjects such as rape and child death, if handled incorrectly you could make the film feel like a tasteless mess that's trying way too hard to feel "mature" and "edgy." These are just a few simple do's and don'ts when handling the subject of animal death in your film.
First and foremost, if a pet dog or cat has is going to die, then it must be motivated by the plot. The death of Old Yeller in "Old Yeller" marks the end of Travis' boyhood innocence and the beginning of his transformation into a hardened adult. The death of John Wick's dog in "John Wick" is what sets the entire plot in motion and pushes John back into his life of death and destruction. The deaths of these animals should motivate/change the characters, convey a theme that runs through the film, and/or set the plot in motion. These two examples also convey how depressing and heartbreaking the death of a pet can be.
Second, you should try to avoid callus animal death or killing a pet without purpose. One of the worst examples of this comes from "The Lost World: Jurassic Park" where a T-Rex is set loose in San Diego and ends up in a suburban neighborhood where it ends up eating a dog. Not only does this grisly death not enhance or motivate the story, but it's played up in the film as if it were a joke the audience was meant to find funny. It comes across (to me at least) as extremely mean-spirited and unnecessarily cruel.
Third, you should try to avoid showing a pet's death on screen. The camera stays on Travis when he shoots Old Yeller and we hear Yeller's cries of pain when he dies. We never actually see Old Yeller get hit by the bullet and fall down dead. A similar thing happens in "John Wick" where we hear the dog's cries and see the body in wide shots later, but we never actually witness the bad guys killing the dog on screen or in close-ups.
Doing this keeps the film from feeling exploitative and tasteless. Not many people actually want to see a beloved innocent pet die in all of it's gory and violent detail. A few horror films do this and it makes me upset. I don't want to watch innocent animals get horribly killed in horror films, just people. Depicting animal death, similar to depicting rape, is something audiences don't want to actually view and is better left implied or depicted in a way that's not exploitative.
The death of a beloved pet is something that's really hard to deal with in the real world which makes it extremely hard to portray effectively in film. These are not concrete rules one has to follow while making a film, but they are good guidelines to keep in mind while writing a film. If you have a film that features the death of a pet it's always important to stop and ask yourself, "why is this important to the story and what does it add". Asking yourself this question before continuing to write will solve 80% of the problems people have with animal death in film.
