While Nightwing is one of the greatest heroes in the DC universe, there was a time when his life might have taken a much darker turn. Being the first Robin allowed Nightwing to make friends all over the DC universe, but those connections came from his relationship with Batman. So if Dick Grayson was never mentored by Bruce Wayne, what would he become?
Batman and Dick Grayson both share the same tragedy; they both had their parents taken from them at a young age by criminals. Because of this, Bruce Wayne became the Dark Knight to prevent anyone else suffering the pain that he felt, which is why he took Dick Grayson in to begin with. Under Batman's tutelage Dick Grayson became the hero Robin and eventually set out on his own as Nightwing. Nightwing is a notably optimistic and well-adjusted hero, but that's because Batman taught him how to deal with the trauma of his parents' deaths. Without that tutoring, Dick's rage turned him into a very different 'hero.'
In Batman #45 by Tom King and Tony S. Daniel, readers are shown a twisted version of the present, created when Booster Gold travels back in time and prevents Bruce Wayne's parents from dying as a gift for Batman's upcoming wedding to Catwoman. This unfortunately has the ripple effect of changing the life of everyone who ever encountered Batman. In a Gotham infected by Joker's madness, Dick became a gun-wielding Batman who makes it his mission to kill any superheroes in the city limits before they become Jokerized. This version of Dick Grayson is ruthless, using bullets and grenades to kill without warning. He even ends up killing Martha Wayne before being shot to death by Bruce.
Batman has always justified his kid sidekicks by arguing that their unique form of rage would otherwise destroy them, and it turns out Dick would have indeed become a homicidal killer in a world without Bruce Wayne. Batman was able to steer Dick and Jason and everyone else he has taken under his wing towards a better path, saving others rather than attempting to hurt those they see as responsible. While it's fascinating that Dick would take up the Batman identity even in a world without Bruce to establish it, it's even more interesting that he's so comfortable using lethal force. In DC's main continuity, Nightwing was shaken to his core when he failed to stop the vigilante Tarantula killing the murderous villain Blockbuster, so to see him tossing grenades without a care shows just how much becoming Bruce's ward helped him cope with his trauma and anger.
Dick Grayson is one of the greatest heroes in the DC universe, he has time and again saved the world, but none of this would've ever been possible if he hadn't met Batman. Bruce Wayne gave Nightwing's rage a focus and ultimately replaced trauma with purpose. Without that focus and direction, Nightwing could've become a masked hero-killer more similar to Batman's rogues than the Dark Knight himself.