Why do you think consideration just occurs because both parties got something out of it? A contract has no consideration if it is illegal, I can pay you 50 bucks to kill my neighbor who I hate. You got 50 bucks, I got rid of my neighbor, you think consideration is present in that contract? Jesus Christ, if a contract violates the law consideration can not exist.
A contract without consideration is not a contract. An 'illegal contract' may have consideration. The example you gave has consideration ($50 in exchange for murder) even though it is an illegal contract (because a contract cannot be used for crime)
I dont want to derail too much, but search for the Peppercorn Principle to read up on consideration.