IMO, from Dick's point of view, Tarantula starts off something like this: vigilante, theoretically wants to save/protect people, but breaks that bat-code of being happy to kill people. In addition to that, she likes him and wants to work with him. He sees the potential to "fix" her, to get her to see why killing is wrong, to either persuade her to follow his moral code, or get her to stop being a vigilante altogether. Either way, she's a potential friend or enemy in his city, which means he has to keep an eye on her, and he feels responsible for what happens to her and for her actions.
It may not be the most common scenario for an abusive relationship, but it's a scenario which is possible for Nightwing to be in such a relationship.
(I understand your complaint about how Dick's friends/family should've intervened, and that any book about Dick should involve him being in regular contact with a wide range of people, and not all isolated and helpless. However, I'm willing to suspend my disbelief when it comes to that aspect at this point. It's my own choice as a reader, and not a judgement on any of yours.)
It's a horrible situation for him to be in, and if he weren't already in such an awful state emotionally, mentally and physically, and he likely would have prevented things from spiralling downward that way, and he would not have let Tarantula take advantage of him in any way as soon as he saw her pointing that gun at Blockbuster.
I wasn't reading comics when this stuff came out, but everything in Nightwing from around that time and all of War Games strikes me as intentionally being one big gigantic trainwreck. It's Murphy's Law gone wild: what can go wrong will go wrong. It was just tragedy after tragedy, and I guess the end result may eventually supposed to have been a catharsis, but I gather that OYL truncated that possibility.