It's extremely common for serial killers to begin torturing and killing small animals while they are children or teenagers. Eventually, that doesn't give them enough satisfaction and they move on to killing people.
In one of the Discworld novels, the Patrician hears off a man who cut a dog's legs off with an axe. He immediately orders Vimes, head of the City Watch, to search the man's house, because anyone capable of such violence towards a helpless animal could do far worse things.
The man is later sentenced to death after the Watch find something in his basement(exactly what is never revealed)proving the Patrician right.