In his pre-realization, Tyler's statements might be seen as Marla being hazardous since she knows too much about the fight club and its transactions. We now know that Tyler regards Marla as a threat since she has the ability to stop Tyler's psychic manifestation (or at least his agenda). Thus, in order to keep her quiet, he tries to kill her.
In addition, it can also be said that Tyler wants to kill Marla because she betrayed him. She discovered his secret identity and reported him to Tony for money. This caused a huge rift between them which only gets worse when we find out what really happened to Marla over time.
Finally, Tyler wants to kill Marla because she doesn't trust him. Even after all he has done for her, she still fears him even though she should know better. This makes him feel unwanted and insecure which causes him to want to hurt Marla by killing her.
In conclusion, why does Tyler want to kill Marla? Because she knows too much about him and his business and she threatens his existence. He decides to take action by killing her so she will be unable to tell anyone about the fight club.
And there's a lot of dualism here, because the narrator hears Tyler's comments differently before he knows he's Tyler. Once he realizes it's him talking, however, his opinions become more personalized and thus less likely to cause harm.
Also, consider that while Marla is indeed aware of the fight club, she's also a major part of it. She helped create the scene that led to Brad's death, so even if she didn't know who Tyler was at the time, she certainly knew what he was capable of. And yet, she still went ahead with their affair despite these facts. This shows that even though she may have been in danger during their first meeting, once she found out it was Tyler who had saved her, she no longer stood a chance.
Finally, remember that this is a novelized version of a real event. Thus, it can be inferred that although Marla wasn't killed or seriously injured due to Brad's death, it certainly didn't help matters any. After all, she'd already fallen in love with someone else. So not only was she in danger because she knew too much, but now that Tyler knew she was involved with someone else, he was likely going to try and kill her.
Marla is a kleptomaniac who is self-destructive and manipulative. Later on, Jack creates a hyper-masculine Tyler who is publicly disruptive, openly honest, and despises consumerism, and so has no need to steal. When Jack's supervisor discovers some Fight Club memorabilia on the Xerox machine, he says: "You don't just copy something like that". To which Tyler responds: "Why not? It's easy. And what's more, it's better than what's real, because this one can be erased."
Also, Marla tries to kill herself by jumping off a building, but saves herself at the last minute. Meanwhile, Tyler throws himself under a subway train to save her.
Finally, Marla runs away from home while still underage, and Tyler has never left Fight Club.
Although they're both involved in Fight Club, they function as separate characters with their own story lines. Even though they are both present at all times, they are not identical twins since they come from different parents and have different personalities.
Tyler dies, and the narrator's mental projection of him ends. Members of Project Mayhem transport a kidnapped Marla to the facility. The narrator and Marla hold hands as the bombs detonate, toppling buildings around them. When everything is done, the narrator finds that he has been spared, but Marla has not.
Fight Club was intended to be a one-time event, but after an audience member brought back proof that some of its concepts had become mainstream culture, Brad Pitt and David Fincher decided to make another film about it. They hired Chuck Palahniuk to write new scenes for the sequel, which were incorporated into the final version. The sequel, called Fight Club 2, was never made.
In case you're wondering why I am asking these questions, here's the answer: No, Tyler Durden and Marla Singer do not die in Fight Club. If you read the book, you will know what happens to them. If you saw the movie, you would have known what happened to them.
Nate takes his response to its logical conclusion by breaking into Tyler's house, stealing all of his weapons, and then startling his victim when he arrives. As a result, Nate takes matters into his own hands and beats Tyler to a pulp, nearly killing him. One may think that this display of violence is for Maddy, but it isn't. This scene is meant to show us that even though Nate is just a puppy, he still has some growing up to do.
Now, about Maddy. In the beginning of the series, we are told that she loves dogs, so it makes sense that her new boyfriend would be one too. However, as the story progresses it becomes clear that she is actually using him as an excuse not to have to get close to anyone. She meets Nate when he rescues her from being hit by a car, but instead of showing any form of affection toward him, she only cares about getting back at everyone who has wronged her.
In the final episode of season one, it is revealed that Nate died protecting Maddy. She had broken up with Tyler, telling him that she never wanted to see him again. Then, after learning that her father had been killed by Anton, she took out her anger on Nate, destroying his body in order to make herself feel better.
So, did Maddy kill Nate? Or was this just another case of someone doing something terrible to get over their problems? Only time will tell.