There are essentially two parts to your question. While you only ask about "good kids" becoming bad, the statement which follows the question speaks to something very different (therefore, it needs explanation).
There are two very different schools of thought on what may make a good kid become violent with no warning. First, one must remember that all people have a part of their brain whose only function is to support life. In this, some teens may react very violently if they are put in a situation where they must rely upon the instinct of "fight or flight." If the child/teenager chooses to fight, the result can be very violent.
Second, the child/teenager may not necessarily become violent with no warning. It may only happen to be that no one has observed the child/teen being violent. If this is the case, the child/teen possesses violent tendencies.
Violent tendencies are not sudden. In fact, tendency refers to the inclination to behave in a certain way. Therefore, violent tendencies are normal and typical under these circumstances. Typically, a child or teen will not, out of the blue, torture an animal.
Given that the torture of animals is linked to the question at hand, many researchers and psychologists have linked children torturing animals with criminal actions later in life. In fact, Jeffrey Dahmer tortured animals when he was younger.
Essentially, good kids can simply have a bad day where they can "lose it." This does not mean that they will be violent from that point out, torture animals, and become criminals later in life. One must also consider violent actions differently. A simple loud outburst accompanied by clinched fists may be considered violent by some and not by others.