Even worse, for the scumbag dog killer, his wife intended the dog to be a focus for him when she died, to stop him from focusing entirely on his grief.
When they killed his dog, he focused his entire attention on them. We all are drawn to violent uncompromising characters who take the law into their own hands, in films anyway, as the suffering is always abstract, but in reality life would be a dystopian nightmare if we all indulged our anger with impunity.