If someone says there is honor among thieves, this means that even corrupt or bad people sometimes have a sense of honor or integrity, or justice, even if it is skewed. ('Honour among thieves' is the British English version.)
This is used as a term to describe someone in a position of authority who is corrupt, someone who will take money in exchange for doing something for the person paying that may be illegal.
Someone who gets a legitimate job which is the opposite of their previous one. E.G a computer hacker who then helps to catch other hackers or an ex-bank robber who then advises banks on security.
This expression indicates the stealthiness of a person over another to gain advantage of the situation. For instance, if two persons are offered some jobs which are vacant, they resolve to go together next day at an agreed time, but one of them, without telling the other, goes earlier than the other and secures the better of the two jobs, he is said to steal a march on the other person.