The following article contains spoilers from "Dark Birth: Part 1" from DC vs. Vampires: All Out War #2, on sale now.
It's still not known how exactly Nightwing became the latest vampire king. However, the first hints have been given in the backup story "Dark Birth: Part 1" from DC vs. Vampires: All Out War #2 (by Emma Vieceli, Haining, and Troy Peteri). What it implies is troubling, presenting the possibility that he may have willingly chosen to become the new king.
If so, then it only cements how far he had fallen, being seduced by power and temptation well before becoming a vampire. Oddly enough though, it seems like his intentions were genuinely good -- still wanting to help others. Unfortunately for everyone else, Dick Grayson's early adventures with DC's vampire community ensured the downfall of his alternate world.
There are still too many unknowns to confirm what actually happened. Nightwing's efforts to protect his sister during a meeting with Bludhaven's vampire population turned out to be a ruse to draw him. Not to spring a trap, but to ask for his help. This was likely when he went down the path to becoming the next vampire king. They requested that he murder Mary Seward, the previous vampire queen.
These vampires didn't reveal much about themselves beyond their team's name -- the Acolyte of Shadows. They claim to be at odds with Mary and her "progressive" vampires, hinting at a strong ideological divide between them, one that could result in bloodshed. This is why they came to Nightwing for help -- to avoid any more loss of life. However, helping them may only have made things worse.
Calling Mary and her followers progressives implies that Mary wants to split from the traditional vampire means of survival, preferring to try and approach humanity more openly and establish better relations. This would conflict with the ideals of a more traditional vampire, who would believe that humans are only meant to be food.
Approaching Nightwing to ask him to commit regicide is hardly the best way to inspire confidence in them. Considering the hints given though, it seems likely that these speculations are true to a certain degree. If so, then this may not even be about preventing more deaths but simply finding a disposable tool to destroy Mary and one that pushes the blame onto someone else.
Whatever happened after this point, however, must have shaped Nightwing. Delving into the vampire world is to risk temptation at every turn, and he seems genuinely committed in the present day to preserving the vampire population. Perhaps he experienced something during this mission that convinced him the vampires were in the right. Or maybe, being around the Acolytes warped his priorities. He might have even been tempted by the power they had. Either way, right from the start, Nightwing was involved in the vampires' politics. What he witnessed at the start is slowly being revealed to readers. It's clear that things are far more complicated than anyone imagined.