No, our whole structure of individual rights is based on he premise that they are endowed by Our Creator.
You really believe on the premise that each one of us decides our own purpose? Suppose a sick person decides his purpose is to slaughter babies? You'd argue that he was violating the babies rights, and of courseheI would be.
But that's only because God endows us with these rights. According to his own lights he'd be fulfilling his self-defined purpose.
You know, I thought you'd bring something like that up! :-)
One's "purpose" in life is a different question altogether from what their moral code should be.