Why did Christopher's father kill the dog in The Curious Incident of the Dog in the Night-Time?

[spoilers]

Whilst married to Christopher's mother, Ed and Mrs Shears the neighbour had an affair, which was the reason Christopher's parents split up (and the reason the Shears split up). Ed and Mrs Shears then began a relationship, but Ed believed that she cared more for her dog than she did for him. After a heated argument, Ed saw the dog and got angry, and so stabbed it with a pitchfork.