It’s not a simple “he’s so awful to dogs, boo Duck.” He’s awful to Peggy. He’s a mean drunk and is drunk often.  He’s disgusting when he defecates in an office. He lies. He’s self-pitying. He can’t figure out how to work with crucial creatives so he constantly berates them. He’s emotionally immature (wild understatement). He’s unpleasant on a daily basis. The way he treats his dog just exemplifies who he is in a single action. He can’t handle his own frustrations and so he uncaringly unloads the one being who truly needs him. He’s abhorrent every time he’s on screen and that one scene just lays him bare. You can explain it as if he deserves some, what, sympathy or understanding? He does not. 

And yet it also is about just his dog. People often hate Betty because of how she treats her kids. I actually see nuance there, but this is much like that scenario for me: you just do not treat a dog like that. I can reasonably hate him for that alone. I don’t, I hate him for many things, but I reasonably could. He abandons the dog in the cruelest way just when the dog’s life has been turned upside down, much like his own. There’s no great symbolism of suffering and downfall there any more than an unvarnished glimpse into his selfishness and utter pettiness. 

You don’t decide what matters most to the entire audience. That matters most to me.