This is one of those things that people tell you all the time. To get ahead. To get friends. To get girls. This maxim strikes me as odd.

A lot of people don't like you for who you are. They like you because you have Stuff, or because you act around them in such a way that they want to be around you, or because somehow in the future, they might be somehow better off by having been friends with you. Many people don't act "themselves" and it helps them to not do so. They are better off by acting the way that they do rather than "being themselves." Others, either consciously or unconsciously, act differently around different people.

What if someone's "self" was a homicidal maniac or a rapist? Is this the message that should be sent to people and ingrained in their minds at childhood? Then, when they are rehabilitated, they should start being themselves again?

All the time, society nudges people this way and that through its reinforcements. Actions and statements have legal and social implication that impel people one way or the other. There are punishments for acting one way and rewards for acting another. Indeed, should a person be themself if it means a life of suffering? And, if there truly is a "self" that one can be, that is left unaffected by the actions of the outside (otherwise "being yourself" would entail your dependence on others' actions, going against the spirit of the statement), then is not any punishment or reward for the actions of someone being themself just a matter of discrimination?

See this nice Onion article on the subject: http://www.theonion.com/onion3917/local_man_ruins_date.html