The phrase was coined by Aleister Crowley, probably the most famous member of the occult society The Golden Dawn (Late 1800s). Crowley established the philosophy of Thelema which works on the principle that every individual should act in accordance with furthering his fundamental or "true self." The idea is not that if you want to do something you have every right to do it at the expense of everybody else but more that if you allow your actions to be governed by your innate spirituality (your will) and not your base desires or your ego then you will move closer to achieving the full potential of yourself with a view to actualisation of the self. (Think of self actualisation as both the ability to see your own life in perfect perspective and to achieve a clear understanding of your own specific purpose on the earth.).
By the time the phrase had become associated with Satanism (1960s) the meaning had changed slightly. Anton LeVay, author of The Satanic Bible and founder of the Church of Satan wanted to emphasise the importance of the individual in his own right instead of as a being dependent on a belief in deity. The subtext now shifted to the idea of man's autonomy guided by his own will and choice and not through his relationship with the Gods. (It is to be noted that Satanists do not in the strictest sense of the word worship Satan. Satanism, as confirmed on the Church of Satan website is an "athiest philosophy".)
Popular culture has always interpreted the phrase as: "I'll do what I and you can't stop me" but this is too simplistic a view. Certainly there is an element of hedonism involved, particularly in the links with Satanism but fundamentally the phrase is not about fuelling the spontaneous pleasure of the individual, it is about having an awareness of will at the most fundamental level and an understanding that life isn't about day to day sensory experiences but about development of the self. As a witch I can certainly subscribe to that and in the Crowley sense I could agree this is a principle that guides my path as a witch.
However, I'll be honest the modern colloquial interpretation of do what I want and don't consider anybody else leaves me cold. Rights in my opinion go hand in hand with responsibility. Nobody exists in a vacumn and therefore nobody can apply such selfish thinking without consequences. When the consequences are riding roughshod over the rights of others its time to find a different philosophy.
Image http://www.deviantart.com/art/Baphomet-51945358 (Maxsamael)