Could Anakin Skywalker argue the insanity defense for the decades he spent as an agent of evil? Could Anakin claim he was legally insane while following the Dark Side, thus not guilty for crimes ranging from murdering Younglings to blowing up Alderaan?
Anakin Skywalker could not Chewbacca Defense himself out of the billions of people he helped kill. A trial of Darth Vader would have a body count near impossible to determine, given it would include every act of evil from cutting off Mace Windu’s arm (and the Emperor killing the Jedi Master); to butchering Tusken Raiders between Episodes IV and V on Tatooine (See Marvel’s Star Wars Darth Vader comic); murdering Captain Raymus Antilles with his bare hands; ordering the deaths of the crew of the Tentive IV, Force killing Captain Lorth Needa and Admiral Kendal Ozzel when court martials were in order; and many other acts done in the name of “The Doctrine of Fear.”
Darth Vader would not be absolved of criminal responsibility just because Govenor Tarkin gave the order to destroy Alderaan. The “I was just there” or “I was just following orders” did not work at Nuremberg war crimes trials and would not be a defense for blowing up a planet. Vader was an active participant in the Empire’s plans for galactic domination, thus would be viewed as being part of the “inner circle” for the crimes of the Empire.
The Insanity Defense is an affirmative defense where Anakin would need to prove by clear and convincing evidence that “as a result of a severe mental disease or defect, was unable to appreciate the nature and quality or the wrongfulness of his acts.” 18 USCS § 17.
Emperor Palpatine spent decades manipulating Anakin Skywalker’s ultimate fall to the Dark Side. These acts included preying on Anakin’s fears of his wife dying and a promise that Palpatine could save Padme from death. All of Anakin’s actions do not appear to be because of a mental disease, but being gullible. Anakin was promised that he could save Padme, which was the reason Anakin Skywalker marched on the Jedi Temple to murder children.
Every death Anakin committed as Darth Vader was a choice. It was not done because the “Dark Side” made him do it, but because he decided to kill others in the name of “Peace, Freedom, and Justice.” While there is no question Anakin was manipulated into becoming Darth Vader, it was a choice Anakin made for himself. Just as it was a choice Darth Vader made to save Luke Skywalker and throw Emperor Palpatine down the Death Star’s reactor shaft in Return of the Jedi.
As much as Anakin could whine that it was Emperor Palpatine’s undue influence that made him turn to the Dark Side, Anakin Skywalker could not prove by clear and convincing evidence he was legally insane during the decades he murdered those in his path after a failed attempt to save his wife Padme.