(This is put before he becomes Joker. So he’s seen as Jack here.)

Jack and you are best friends, both of you hated your fathers, both of you had horrible home lives, so you just hung out together.

You’d always been there for him and so you were the main person he trusted. One day he kills someone out of anger, but he liked it, and he showed you.

After he showed you, he asked you something.

“What do ya think?”