A person who attacks a network or system with the intent of covering up or finding holes in security; unless you offend them. Then they attack it with the intent of destroying data, or wrecking the system. A combination of a Hacker and Cracker. A hacker is for good purposes (by definition) and a Cracker has malicious intent.
A business hires a Chacker to find holes in security, but then tells hime he won't be making as much as promised. He then finds all of the holes, exploits them, and makes them bigger and easier to get through.