The joker in the movie even challenges basic morality and proves that threatening and desperate times completely remove ordinary morality we follow in our day to day life and makes us act purely on our deepest desires or instincts. This again brings out the question “Is Morality really something absolute/true or is it just something we have been conditioned to believe in?”
There is another dialogue in the movie where he quotes “People are their true selves only just before they die” since they stop acting out of all the created concepts and act purely on instinct at such times. But it is incorrect to say this too. If you create a life threatening situation for someone, his only priority may become saving his life or his deepest beliefs and conditioning may take over. But this too is not who we really are. We are truly just energy. Everything else is part of the situation and our conditioning, level of awareness, level of understanding etc. The nature of reality is still unclear and this theory is just a small fragmentary conjecture in the whole picture.