More info courtesy of Pet-Abuse.com:
According to a 1997 study done by the Massachusetts Society for the Prevention of Cruelty to Animals (SPCA) and Northeastern University, animal abusers are five times more likely to commit violent crimes against people and four times more likely to commit property crimes than are individuals without a history of animal abuse.
Many studies in psychology, sociology, and criminology during the last 25 years have demonstrated that violent offenders frequently have childhood and adolescent histories of serious and repeated animal cruelty. The FBI has recognized the connection since the 1970s, when its analysis of the lives of serial killers suggested that most had killed or tortured animals as children. Other research has shown consistent patterns of animal cruelty among perpetrators of more common forms of violence, including child abuse, spouse abuse, and elder abuse. In fact, the American Psychiatric Association considers animal cruelty one of the diagnostic criteria of conduct disorder.
More recently, high school killers such as 15-year-old Kip Kinkel in Springfield, Ore., and Luke Woodham, 16, in Pearl, Miss., tortured animals before embarking on shooting sprees. Columbine High School students Eric Harris and Dylan Klebold, who shot and killed 12 classmates before turning their guns on themselves, bragged about mutilating animals to their friends.