If you were a lawyer and Lennie were your client, how would you make your case for his innocence in the killing of Curley's wife?
In Steinbeck's Of Mice and Men, Lennie kills Curley's wife in chapter five. Even the best of lawyers would have a difficult time defending Lennie's behavior, but certainly the man's mental condition would be used in his defense. The death is most certainly accidental, since we know that Lennie often doesn't know his own strength. It takes several men to restrain him after he crushes Curley's hand in chapter three. He kills his puppy at the beginning of chapter five and he is positively deadly to any other small animal he gets his hands on. George realizes that the death of Curley's wife, no matter how inadvertent, is the last straw. He is probably right in putting Lennie down, just as Carlson had put down Candy's dog. In the 1930s, Lennie would have been chained up, thrown in a cell and treated harshly. He would not have understood his treatment.
A good lawyer would most certainly try to plead insanity. He would argue that Lennie was mentally challenged and wasn't in control of his actions. The lawyer might use psychologists to diagnose Lennie with a mental disease. Doctors could administer a test (such as the "irresistible impulse" test) which would prove that Lennie suffered from a mental disorder preventing him from appreciating the criminality of his act.
This defense may work because the reader knows that Lennie has a problem. He can only remember things that George tells him. He is obsessed with petting soft things. He also suffers from hallucinations like the ones in chapter six when he is berated for his conduct by an imaginary rabbit and his Aunt Clara, summoned from the dead. The delusions never focus on what he actually did wrong, but on how George will react. Although this defense could very well succeed, Lennie would probably still be locked up. At the time of the story, it would not have been a pleasant experience for Lennie as mental hospitals were often primitive and brutal.