Since the beginning of the story I tried to guess who killed the dog because my teacher said that it was someone unexpected, then I thought Sioban killed the dog, but she doesn't anything about Mrs. Shears or Mr Shears. Then I was thinking maybe the dad, but when he said to Christopher to stop investigating about the "bloody dog," it was a little obvious that he killed Wellington, the dog. Also, when Christopher's dad was talking about how his mom is in the hospital, he didn't give much detail about her and he refused him to accompany her in the hospital, I thought that maybe the mom is still alive, but is living somewhere else. Later on, Mrs. Alexander, the neighbor told Christopher that his mom was having sex with Mr Shears and it's the reason why Christopher's father killed Wellington. I would be Christopher avoiding his father because he killed the dog and it meant that his father had an ability to kill his own son.