What did christopher's father tell him what was his reason for killing the dog?

In Mark Haddon’s novel "The Curious Incident of the Dog in the Night-Time," Christopher's father reveals that he killed Wellington, the neighbor's dog, out of anger and frustration after a heated argument with Mrs. Shears, the dog's owner. He believed that the dog was a source of conflict in his life and thought it would resolve the tension. Christopher struggles to understand this justification, as it goes against his moral beliefs about the treatment of animals.