Came to be....
The Joker came to be over the length of this film.
Beginning = imbalanced, medicated, abused.
Near-ending = balanced, medical-free, in-charge.
Remember what he told whats-his-face (before he got a face/neck full of scissors)…
"My mom died... Im celebrating... off my meds... feeling good"
This is (by his own admission) the first time he's felt good in his whole life!
His meds were keeping him just sane enough to get barely by on a meager existence.
Off his meds, he "realized" himself... he was the Joker, waiting to be born.