FBI Special Agent Alan Brantly believed that some offenders kill animals as a rehearsal for killing human victims. Cruelty to animals is mainly used to vent frustration and anger the same way firesetting is. Extensive amounts of humiliation were also found in the childhoods of children who engaged in acts of cruelty to animals. During childhood, serial killers could not retaliate towards those who caused them humiliation, so they chose animals because they [animals] were viewed as weak and vulnerable. Future victim selection is already in the process at a young age. Studies have found that those who engaged in childhood acts of cruelty to animals used the same method of killing on their human victims as they did on their animal victims.
Wright and Hensley (2003) named three recurring themes in their study of five cases of serial murderers: As children, they vented their frustrations because the person causing them anger or humiliation was too powerful to take down; they felt as if they regained some control and power over their lives through the torture and killing of the animals; they gained the power and control they needed to cause pain and suffering of a weaker, more vulnerable animal – escalating to humans in the future.
In a study of 45 male prison inmates who were deemed violent offenders, McClellan (2003) found that 56% admitted to having committed acts of violence against animals. It was also found that children who abused animals were more often the victims of parental abuse than children who did not abuse animals. As previously stated, animal cruelty was a way for the children to feel as if they were retaliating against those who abused, frustrated, or humiliated them.
In a 2004 study, which considered not one-off events but patterns of repeat violence, Tallichet and Hensley found a link between repeated animal cruelty and violence against humans. They examined prisoners in maximum or medium security prisons. However, over-generalizing possible links between animal violence and human violence can have unwanted consequences such as detracting focus from other possible predictors or causes.
https://en.wikipedia.org/wiki/Macdonald_triad
