In The Dark Knight Returns, Bruce laments the death of Jason Todd, and decides to take Carrie Kelley under his wing (no pun intended). Dick Grayson, Age Twelve, may be the reason why Bruce actually develops some humanity to his other Robins.
The whole story could be viewed as Frank Miller sitting down and going "would actually happen if a rich 8 year old boy mentally never grew up after his parents died in front of him and became obsessed with his 8 year old ideals of justice and law?" Going so far as to become a vigilante, believing the cops are either too inept or corrupt. Notice how he acts towards Grayson when he wants to impress him, like a kid showing another kid his favorite toys. Notice how impulsive and child like he is? Because he's still Bruce Wayne, Age Eight, crying for his mom and dad, but they won't come.
As pointed out in Linkara's review, he points out that Batman mentions "having [his] eye on [Dick] for some time" and mentions "other candidates for the job", which would seem to indicate he had been planning to bring Dick in on his crime-fighting well before his parents were murdered. Linkara further interprets Bruce's snarls that "he'd have waited years" to imply he would have abducted Dick and possibly killed his parents himself to get him to join him.
Linkara also pointed out that we never see Batman giving Hal Jordan his ring back. The Goddamn Batman with a Green Lantern ring...oh dear...
A horrifying thought occurs when combined with the above Fridge Brilliance and the Fridge Horror about the Green Lantern ring. Batman is mentally eight years old, afraid of nothing, fanatically dedicated to his job, unable to work with others, and has no qualms about killing people. Dark Nights: Metal shows exactly what happens when a sociopathic, damaged, murderous Batman acquires any sort of power.
Batman smashes the Batmobile through cop cars, delivers a flying kick hard enough to break glass to an officer's jaw, and sets criminals on fire with thermite. There is simply no way these people are living through the night, and if they do they are certainly going to be disabled for life at least.
So, why did the Joker kill Dick's parents? Granted the series was Cut Short so Miller never got a chance to answer that question, but with everything else going on in the series, there is nothing to suggest the Joker has anything to gain from randomly shooting two trapeze artists. Then again, this is the Joker; him shooting random innocents For the Evulz would be valid characterization.
If the other remarks are foreshadowing, it might mean something completely unexpected and even worse than previously thought: Batman had him do it.
Batman's strategy for dealing with Green Lantern in issue 9 was to make sure everything in the room was yellow (even the lemonade). What would he have done if Hal had, oh say, gone outside, grabbed a car, and chucked it at Batman? I doubt Batman would've been able to make sure all the cars outside were painted yellow as well.