
Understood.
I actually think efforts to analyse The Joker as a real human being are futile, because he simply isn't written that way. He is, in a sense, a classic comicbook superman (small "s") in that he has been the victim of some great injury from which he has derived no detriment and has instead been strengthened. With The Joker, however, this occurs purely in the psychological sphere, where he is completely invulnerable. His "insanity" is not of a type that we would usually recognise, because his cognitive abilities or emotional intelligence are completely unimpaired by his sadism or his lack of empathy. Even these are subordinate to his intelligence: he doesn't
need to murder people in the way that Zsasz does; he appears to be able to sit on his hands calmly enough and await the best opportunity. He is also almost entirely without fear. I think, in essence, he only really has a tight trio of emotions- cruelty, narcissism, and disappointment (which is the most he suffers when his schemes are thwarted). The mirthful cackling seems to be a product of the first two.
I think that is a rational, choice however. Batman usually uses violence as a means to an ends. When he goes too far, it is usually a rational manifestation of rage.