The most common and used argument in trials

It's not surprising to say the least. In almost all cases murder trial, homicide attempts. It just is amazing that laywers and the defendants always uses same kind of defense: INSANITY!. Especially if they think that the case has nowhere to go and it's obvious that a conviction will be the outcome. Take the case of the sniper killer.
U.S. News: Lee Malvo's brainwashing defense. Part of the report says -- The plain facts are now blurred by the defense argument that Malvo should be considered not guilty by reason of insanity. In the end, even if proved, mental illness may not save Malvo from a guilty verdict.

Remember latter part of 90's when school killings were occuring frequently and the people(teenagers) charged would always used the defense of having mental illness, abusive parents, denied love and attention hencethe need to do what they did. Or how about the Menendez trial. They used reason of insanity as their defensefor them not to be convicted. It didn't work out.

Same thing will probably happen to sniper killer.