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I don't think this story will work, or your solutions are viable, for the reasons you wrote. You violate reader trust from the beginning, the first person narrator knows he intends to kill everyone...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/33337 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/33337 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
I don't think this story will work, or your solutions are viable, for the reasons you wrote. You violate reader trust from the beginning, the first person narrator **_knows_** he intends to kill everyone and **_knows_** he is the murderer, how can you possibly relate the murder of the first person, with him telling the tale, and NOT revealing he is the killer? Basically you have an untrustworthy narrator, and to be fair to the reader you must signal this untrustworthiness early in the story, before any killing, but then again, because your narrator is untrustworthy they will figure out he is the killer. If I were writing this story, I would begin with the admission of guilt. "I intended to kill six, and six died." And show the squirming worms in the head of the narrator as he seduces his victims onto the boat or whatever, and misleads the others into thinking, say, his girlfriend Alice or one of the others is the murderer, leaving them for last, and then, when it is just him, Jack and his girlfriend, arrange to make it look like Jack came to kill HIM and he kills Jack in self-defense. Okay, Alice, we're safe ... (until I come for you). Your twist can be, Alice already figured out it IS him, because she hacked his amateur passwords a year ago, when she first moved in, and she has read all his hackneyed horror fantasies and recognized his fictional serial killer in the first murder! He doesn't know that when Alice rushes to hug him, because they are safe, and he basks in the knowledge he has saved the best torture for last with her, until the moment she slices through his femoral artery with a butcher knife. Then, while he bleeds out on the deck, Alice explains it all to HIM. Then, he can say, "I intended to kill six, and six died." He just did not expect to be one of them. And your narrator can be truthful throughout.