Black hat hackers
    
A black hat hacker, as the name suggests is the polar opposite of a white hat hacker in terms of both intention as well as methodology. They violate a network for malafide intentions for monetary and personal gains. They are the illegal communities who fit the commonly perceived stereotype of computer criminals.

They gain access into a system and steal or destroy the information or modify the same. They may tweak the program in such a way that it is rendered useless for the intended users. When they notice a weak spot or vulnerable area in the system, they take control of the system by way of such weak spot. They keep the proprietors, authorities and the general public in the blind regarding such vulnerability. They do not make any attempts to fix the lacunae unless their reign faces a threat from a third party.