I disagree, if Joker were REALLY insane, there's no way he could focus enough to make all those plans and gadgets.
He's not insane, just very focused in an unusual way.
Pretty sure Joker is aware of what is generally considered "right and wrong"
Insanity is no longer considered a medical diagnosis but is a legal term in the United StatesMost courts accept a major mental illness such as psychosis but will not accept the diagnosis of a personality disorder for the purposes of an insanity defense. The second question is whether the mental illness interfered with the defendant's ability to distinguish right from wrong. That is, did the defendant know that the alleged behavior was against the law at the time the offense was committed.
Additionally, some jurisdictions add the question of whether or not the defendant was in control of their behavior at the time of the offense
http://en.wikipedia.org/wiki/Insanity
This may disqualify Sentry from an insanity plea as well.