Animals are innocents and deserve our compassion. Therefore they are used as shorthand, particularly in film which always must be kept moving, to mark good and bad, right and wrong, moral and asshole. It doesn’t matter what else you do in the story, if you are kind to a dog, the audience will pretty much follow you anywhere.
Forget the good, good guy –you know the guys who are already morally perfect but fighting against a single vice –Ripley from Alien or George from Kuffs. These guys are kind to animals against reason, but we expect that from them. Instead think of the bad, good guys –the guys who are truly shitty human beings but because they cling to a single virtue and stand next to shitier humans beings we root for them –Eli kills nine guys in the opening of his movie but is nice to a rat: good guy. Riddick kills everything all the time but the scary space dogs are nice to him and he’s nice to them back: good guy. Martin Blank, John Cussack from Grosse Point Blank is a paid assassin but what separates him from the evil Dan Aykroyd? Cussack would “never hurt and animal because he has scruples.” Then (off the top of my head) there’s Hannibal, Brotherhood of the Wolf, Dragon, The Crow, Last of the Mohicans, Reanimator, Silverado. But the one that strikes me most is Equilibrium. Anyone remember this Christian Bale Sci-fi piece of awesomeness?
Allow me to set the stage. Bale starts off the story as a bad guy –an enforcer for a corrupt and fascist government. He kills people because they keep art. But then he has a moment, helped by some sci-fi exposition, and he becomes a good guy. How do we know he’s really a good guy? Well a couple of his bad guy buddies show up with a dog. Now bad guys are supposed to kill dogs because they serve no purpose in the new society. And the bad guy buddies suspect Bale has gone native, so they want him to do the bloody deed. Now Bale has a choice. He can shoot the dog, allay suspicion, and then go save the world without every cop on the beat chasing his ass down, or he can shoot the two dudes, who until five minutes ago were his best friends in the whole, entire world. What’s he pick? Of course he shoots the dudes. Come on. They were people, and they deserved it. After all, it’s not like they had been subjected to a brutal brainwashing death machine that literally took control of all their thoughts and feelings. Oh wait…. That doesn’t matter. They weren’t cute and furry.
The point is kill, rape, pillage your black little heart out forever, and if you do it with a pet all will be forgiven. Maybe that’s why pirates carry parrots.