The following article contains spoilers from Future State: Gotham #16, on sale now from DC Comics.
Whatever Nightwing saw in the future must have been truly terrible because he has upped his dose of brane, a performance-enhancing drug. In Future State: Gotham #16 (by Dennis Culver, Geoffo and Troy Peteri) while fighting off his family members, Nightwing took a dangerously large dose of the drug. It ended up heightening his mental abilities, not only giving him what appears to be a form of flight, but also telepathic abilities.
While these abilities might come in handy, especially considering Batman 666 most likely possesses dark powers from his time in Hell, there is a cost. Overdosing on any kind of drug is dangerous, and brane is no ordinary chemical mixture. The full consequences of its uses are still largely unknown, and it's not clear whether Nightwing is even fully in control of his actions. If his behavior is any indication, however, someone is going to die soon, and he might have just dug his own grave.
His first major dose of brane nearly killed him, subjecting his mind to intense visions of the future. It's still unclear whether what the brane showed him was accurate, or if he was interpreting the vision correctly. Regardless, Nightwing has come to believe that only he knows how to save the world, and has been relying on the drug more and more. Now, he seems to have gone past the point of no return.
While advantageous for the war ahead, there are a lot of unknowns that only promise to make things worse. For starters, it is not clear whether Nightwing is guided by his belief in his visions or if the brane is simply messing with his thought process. By the end of the issue, he behaved as though he was the only Batman that mattered and claimed he "would never die".
Now, there are some possible explanations for this. One of them is that Nightwing was not talking about himself, but simply about the concept of Batman. However, it is just as likely -- if not more -- that he actually believes that he can't die. When people gain significant power they can ride that high for a while before coming down. The only difference here is that Nightwing can choose when to come down from his high, but he's unlikely to do so.
The brane may very well be speaking for him at this point, with the effects of the drug overriding his usual self. If so, then Nightwing is no longer acting with the intent to stop whatever apocalypse is coming to Gotham. He's simply living out a delusion that was never real, to begin with. Assuming he can survive the battle against his family, the drug might still kill him anyway.