I know that Vincent Bugliosi (one of the good Vincents!) has been saying for years that one of the ways he was prepared to fight an insanity defense in the Manson case was by pointing out places where they took actions to evade detection. An insane person will not cut telephone lines, was his premise. The fact is, lawyers can't get into defendants' heads. Even if prosecutors are permitted to have the defendant given a psychological examination, that is necessarily of very limited scope. (Sanity and competence.) All legal determination of motive is a guess, and just because a jury doesn't believe a prosecutor about intent doesn't necessarily mean they won't convict. If they don't believe the prosecutor about intent, they cannot convict. Using Occam's Razor might help work out intent, but its failure with motive shows that it isn't foolproof when it comes to human emotion.