You are looking for a rational answer and the simple answer is - there isn't one. People perform these mindless acts of cruelty for no concious reason, if you asked them why they did it, they would tell you they don't know why, and mean it. People who do these things have no sense of morality of concience, so expecting one is futile, and merely causes more distress to the rational thinker. They may get punished, they may not - society is a long way from perfect - we just have to do our best with what we have.