Either

"Guard Dogs represent the deadliest level of canine training. These animals either walk with a sentry, or patrol alone in an enclosed space. Their function is to apprehend and neutralize any human intruder. They do not stop biting when the suspect stops resisting; they stop only when the human stops moving. They are likely to be trained to go for the throat or genitals. Guard dogs are trained to kill and maim. A guard dog is so vicious that it will usually obey only a single handler. ... The only legitimate use of a guard dog is in wartime, or when guarding an area so sensitive that human intrusion could result in awesome public danger, such as a nuclear weapons facility."

or

"Attack dogs have been trained to sink their teeth into people on their master's command, or when they observe their master under assault. Once resistance from the suspect ceases, a true attack dog will let go of him. It will do the same on command, no matter how excitement-charged the atmosphere, if it has been properly trained and selected. Normally, the dog will only bite if given the proper command, or if the animal sees its owner or a family member under attack.The attack dog is at the maximum level of obedience training. After the master has ordered it to put a suspect on point, the dog can be called back, and even ordered to "make friends." It feels no personal animosity toward the person it is ordered to attack; it is a canine technician, doing a job on the orders of its human boss"

Probably the latter, but sometimes going for the kill is fun.