Re: If the author wanted a "killing is always wrong" shpiel, he picked the wrong villains.
For the purposes of law in the US, the Joker would not be considered sufficiently insane to be either (1) incompetent to stand trial or (2) not guilty by reason of insanity. To be sufficiently insane for either of those, you need to be so gone that you have no idea what you're doing at all (i.e. killing someone because you think they're a rampaging space alien about to eat your eyes) or you think you're being ordered by G-d or something.
The Joker knows exactly what he's doing, and he knows it's considered wrong and illegal. He's just a sociopath. He would have been executed and/or put in jail for life long ago.