Subverted: Bob prepares to shoot the dog, but Alice stops him at the last minute, and convinces him to activate it some other way.
Double Subverted: ...Then Bob shoots the dog anyway, when he finds he's running out of time.
Bob shoots the dog, then beats its carcass into pulp, tears out its intestines and wraps them around his head and starts splattering blood all over the place, all the while sobbing and raging at the heavens that this for the greater good!
While Bob spends an hour ruminating back and forth over whether to be good or logical, the dog rolls its eyes, pulls out a revolver and shoots Bob instead. The MacGuffin activates. The dog saves the cosmos and and is hailed as a hero.
Zig Zagged: Bob seemingly shoots the dog. Then it turns out it's just playing dead. But actually he meant to shoot it, he just missed. He tries again, but it survives. The reason it survived is that it's a zombie, so Bob eventually resorts to Removing the Head or Destroying the Brain. Then it turns out that doesn't even work on this kind of zombie...
Averted: Bob doesn't shoot the dog.
Enforced: The writers need to show that things have become extremely serious, so they have Bob need to shoot the dog to continue his quest.
Lampshaded: "We're going to have to shoot poor Fido now, aren't we?"
Invoked: The Big Bad sets up a situation where the only way out is to shoot an innocent puppy.
Exploited: The Big Bad uses video of Bob shooting the dog to break public support for him.
Defied: Bob takes steps to keep from having to shoot any dogs.
Discussed: "Just watch, we're going to have to shoot a dog or something."
Conversed: "They're shooting a dog? Are you serious?"
Bob's shooting of an innocent dog makes him a pariah in the eyes of the world, leading to his downfall.
Sooner or later, it becomes evident that Bob's continued shooting of dogs are not because it is necessary: it's just easier.
Reconstructed: Bob's shooting of the dog makes him a pariah, until everyone learns exactly why it was necessary to do so.
Played For Drama: Bob is forced to kill his only companion from all these years, to ensure millions don't die.