The boy's sister, tried to intervene to stop him from hanging the terrier mix, but he battered her and drove her off, say cops.
"I was mad at the dog, so I killed it," the unidentified juvenile casually told investigators after the Friday incident in Salinas, police said.
"I wanted to see it die," the boy purportedly boasted, the 'New York Daily News' reported.
Police found the canine corpse after the boy's sister called 911 and blurted out: "He's hurting the dog,".
"She was trying to intervene, and he battered her," Officer Miguel Cabrera told The News.
"So she left the apartment with a second dog, trying to protect it."
Investigators visited the address for a welfare check a short time later and could hear a loud TV blaring inside, though nobody answered, he said.
They were able to gain entry into the building with a pass key, and officers found the motionless 12-pound terrier mix hanging by its collar from a bedroom door handle, police said.
The boy came out of the same bedroom and "spontaneously" admitted his role in the heinous act.
He was booked in Monterey County Juvenile Hall for battery on his sister and felony cruelty to an animal, Cabrera said.