http://www.crimelibrary.com/serials/what/whatevents.htm
Animal Cruelty
These secret compulsions are seen as the seeds to greater mayhem. "Violent acts are reinforced, since the murderers either are able to express rage without experiencing negative consequences or are impervious to any prohibitions against these actions. Second, impulsive and erratic behavior discourages friendships," increasing isolation." "Furthermore, there is no challenge to the offenders' beliefs that they are entitled to act the way they do." (Ressler, et al, Sexual Homicide) "All learning, according to Ressler, has a "feedback system." Torturing animals and setting fires will eventually escalate to crimes against fellow human beings, if the pattern is not somehow broken.
Torturing animals is a disturbing red flag. Animals are often seen as "practice" for killing humans. Ed Kemper buried the family cat alive, dug it up, and cut off its head. Dahmer was notorious for his animal cruelty, cutting off dogs heads and placing them on a stick behind his house. Yet not all serial killers take their aggressions out on pets. Dennis Nilsen loved animals, particularly his dog Bleep, whom he couldn't bear to face after being arrested for fear that it would traumatize the dog. Rapist torturer and murderer of eight, Christopher Wilder, had made donations to Save The Whales and the Seal Rescue Fund.