A defense lawyer's job is to provide the best defense he can for his client.  It is the prosecutor's job to prosecute him.  

If you don't like insanity defense, then that is something to take up with the legislature.  It's their job to make or change laws.  The lawyer's  (on both sides) job is to argue the case under the laws given by the legislature.  

Nobody should be allowed to graduate high school without enough of a basic understanding of civics to "get" the idea of how that works.