Why do people dehumanise criminals and bad guys?

Do you believe this is part of the reason why history is allow to repeat as it does, because humans do not recognise that one of their own is responsible for the bad actions, choosing instead to call the people to blame "inhuman" or "monsters"? I have also notice they often place the blame for a large group on one person so that they can dehumanise the person and avoid attributing his or her actions to the rest, who may be equally to blame or even more so.