1. Today I pull the trigger on a guy who killed twenty people yesterday.
It is not legal for me to do this. Nor is it lawful for me to do this. Where's the difference?
2. Government indicts the killer, tries him, sentences him to death and executes him. This is both legal and lawful. Where's the difference?
I Googled for the question and came up with this on Wiki.answers:
"Legal" looks more to the letter [form/appearance], and "Lawful" to the spirit [substance/content], of the law. "Legal" is more appropriate for conformity to positive rules of law; "Lawful" for accord with ethical principle. "Legal" imports rather that the forms [appearances] of law are observed, that the proceeding is correct in method, that rules prescribed have been obeyed
According to that, it looks like "lawful" is used when referring to the "spirit" of the law, or to some theory of "natural law" or common law. "Legal" means the letter of the law, as interpreted by government courts. Does that seem about right to you? If so, then I was right the first time, in my use of "legal."
Recommended reading: The Most Dangerous Superstition by Larken Rose