The difference between law and ethics could fill volumes. In fact, the legal codes of many countries do (Westlaw, 2010) (LawInfoChina, 2010). While disagreements about ethics continue, this writer hopes that they will be divorced from mysticism in the foreseeable future and so be addressable with rigor and honesty. Legal differences, on the other hand, are so notable from jurisdiction to jurisdiction as to be occasionally irreconcilable. This reflects local culture and mores to some extent, but it is also a result of the different political and economic systems themselves.
What inspires unethical behavior is often simply animal nature. While it's frequently labeled as human nature, this designation is highly debatable as we don't even really understand how cognition works (Robinson, 2007); how can we possibly understand all the behaviors that come from and support it? However, there is plenty of empirical evidence that animals are often greedy, callous and rapacious. And while there are some anecdotes of selfless and sensitive animal behavior they are frequently unsubstantiated or explicable by resort to simpler justification. So ethical considerations are, apparently, largely a corollary to heightened self-awareness. Therefore intentionally unethical behavior might be said to be an absence or direct contravertion of this awareness, or some related belief that doing the wrong thing is acceptable.
What inspires illegal behavior is more complicated. Because law can be so voluminous and convoluted the reasons to violate it must be at least as much so. Causes might range from a similar ethical failure to control asocial desires to outright psychosis. They might include the perception of otherwise insurmountable financial hardships or a cognitive dissonance that has proceeded so far as to condone the taking of human life. There may be laws that were implemented from 'ethics' that aren't actually meaningful but which have become ingrained in the patterns of social conditioning. For, what sets the criminal act apart from the merely unethical in the liberal tradition might be summarized by Mill (1869): “the only part of the conduct of any one, for which he is amenable to society, is that which concerns others.“ The notion of a victimless crime is therefore not terribly valuable. In a world where innumerable victims suffer without ever receiving anything approaching justice it is probably more sensible to concentrate our resources on those crimes which do have victims.
So the simplest litmus test of unethical versus illegal behavior might be the existence of the victim. If liberalism continues toward the Millian ideal (Burgess-Jackson, 2004) this could well become the case. But even this proof is very much complicated by the perception of the victim, if they still exist, and their ability to communicate how they were violated, or even if.
Burgess-Jackson, K. (2004) 'Our Millian Constitution', Notre Dame Journal of Law, Ethics and Public Policy, Vol. 18 [Online]. Available from: http://www.uta.edu/philosophy/faculty/burgess-jackson/Burgess-Jackson,%20Our%20Millian%20Constitution.pdf
LawInfoChina (2010) [Online]. Available from: http://www.lawinfochina.com (Accessed: 6 June, 2010)
Mill, J.S. (1869) On Liberty [Online]. Available from: http://books.google.com/books?id=RbkAAAAAYAAJ&printsec=frontcover&source=gbs_v2_summary_r&cad=0#v=onepage&q&f=false (Accessed: 6 June, 2010)
Robinson, D.N. (2007) Consciousness and Its Implications. Chantilly, VA: The Teaching Company
Westlaw (2010) Database Directory [Online]. Available from: http://directory.westlaw.com (Accessed: 6 June, 2010)