Assassin is a word used for committing murder of a public figure in a surprise attack. History is full of such incidents. Many political leaders and man in power were dramatically murdered. This is not the end, modern age is also full of such incidents, and difference is the tools used for the assassination. The purpose behind this is only political rivalry and lust of power.
Villains are the most important role in any movie, play or drama. We can say that this role is most wanted in societies, without any villain no hero will exist. Gangsta are anti social, uneducated and illegal activist group. Some groups use this name for musical band, fashion and social groups. In this economic slump criminals are providing employment to the law and order institutions. It is funny but a reality as well. Society without crime has less police staff. Mafia is a term used for criminals after 18th century. Mobsters are associated with groups of criminals.