WARNING: The following article contains spoilers from DC vs. Vampires #6, on sale now from DC Comics.
Whenever considering the question of who is the most dangerous member of the Bat-Family, most fans defer to Batman. While this is a fair assessment, DC vs. Vampires #6 (by James Tynion IV, Matthew Rosenberg, Otto Schmidt, Simone di Meo, Danielle di Nicuolo, Rex Lokus, and Tom Napolitano) reveals that this may actually be a premature conclusion.
The issue reveals that Nightwing is the new king of vampires, and the revelation proves that Dick Grayson is a greater threat to DC's heroes than Batman ever could be. The reason for this ties into how each man is viewed by the world around him. While the Caped Crusader comes across as dark and scary, Nightwing is easy to get along with and one of the most beloved figures in the superhero community. As such, people lower their guard around him, and it is that familiarity that makes them vulnerable.
The revelation that Nightwing was responsible for all the recent deaths caused by the war between humans and vampires was a blow to everyone. Batman paid for it with his life when he realized too late that one of his own was working against him. Even then, his immediate reaction was to turn towards the younger Robins to try and figure out which of them was the enemy. This is telling all on its own. Even in his worst nightmares, Batman would never doubt Dick Grayson. He was the best of them, the proverbial light in the darkness that always shined through even the most trying times.
Unfortunately, Nightwing has taken full advantage of the trust he has attained. Being Batman's very first sidekick, he was fully aware of how the Dark Knight operated better than any of the other heroes. Equipped with this knowledge and foresight, he anticipated that once Batman got wind of the rising vampire threat, he would take precautions to ensure his circle of trust had not been breached. From there, it was easy to get what he needed to pass all the tests Batman had set up to identify whether any of them had been turned.
Nightwing used the same tactic against all the heroes he turned. In the brief psychic conversations he had with Green Lantern, there was still a level of familiarity between the two. They spoke as friends, not as members of an established hierarchy. Dick's amicable personality is what allows him to work so easily with other heroes and that in turn gives him a level of access that Batman has never had.
Ironically, this means that Batman's policy about limiting who he trusts was right in the end. It wasn't a supervillain who destroyed the Justice League, but rather a friend who proved capable of deceiving the World's Greatest Detective.