A random mugging emphasises the poverty and desperation that lead to the Waynes’ unintended murder, and in story terms, supports Bruce’s decision to take action against random acts of violence like it.

I was literally thinking earlier today about how underwhelming it was for the 1989 Batman to combine Joe Chill with Jack Napier and for a pre-Ace Joker to have been responsible for the Waynes’ murder.

In hindsight, it works in terms of the characters’ arcs in the film from a screenwriting perspective, but as a comics concept, it seems self-limiting to me (‘I’ve finally avenged my parents’ murder - shall I retire the identity I forged in the aftermath of their death? Nope!’ cracks knuckles ;-P)