There are many different reasons why individuals abuse animals. Animal cruelty covers a wide range of actions (or lack of action), so one blanket answer simply isn't possible. Each type of abuse has displayed certain patterns of behavior that we can use to help understand more about why people commit the crimes we encounter today.
Animal cruelty is often broken down into two main categories: active and passive, also referred to as commission and omission, respectively.
Passive Cruelty (Acts of Omission)
Passive cruelty is typified by cases of neglect, where the crime is a lack of action rather than the action itself - however do not let the terminology fool you. Severe animal neglect can cause incredible pain and suffering to an animal.
Examples of neglect are starvation, dehydration, parasite infestations, allowing a collar to grow into an animal's skin, inadequate shelter in extreme weather conditions, and failure to seek veterinary care when an animal needs medical attention.
In many cases of neglect where an investigator feels that the cruelty occurred as a result of ignorance, they may attempt to educate the pet owner and then revisit the situation to check for improvements. In more severe cases however, exigent circumstances may require that the animal is removed from the site immediately and taken in for urgent medical care.
Active Cruelty (Acts of Commission)
Active cruelty implies malicious intent, where a person has deliberately and intentionally caused harm to an animal, and is sometimes referred to as NAI (Non-Accidental Injury). Acts of intentional cruelty are often some of the most disturbing and should be considered signs of serious psychological problems. This type of behavior is often associated with sociopathic behavior and should be taken very seriously.
Animal abuse in violent homes can take many forms and can occur for many reasons. Many times a parent or domestic partner who is abusive may kill, or threaten to kill, the household pets to intimidate family members into sexual abuse, to remain silent about previous or current abuse, or simply to psychologically torture the the victims, flexing their "power".