Alien: Covenant leaves the question unanswered, but did Michael Fassbender's David 8 really kill his android "brother" Walter? There was much rejoicing in the Alien fanbase when Ridley Scott signed on to helm a prequel, which was then dubbed Engineers. This would have revealed how the derelict spaceship containing the Xenomorph eggs ended up on LV-426, but over the course of development, Prometheus became more of a standalone with few direct Alien ties. While the movie was undoubtedly ambitious and visually arresting, a tonally confused screenplay and characters making poor choices to push the plot forward made it a disappointment.
It also lacked the central Xenomorph creature, with Scott later revealing he felt the monster was "cooked" due to overexposure in sequels and other media. One of the most acclaimed parts of Prometheus was Michael Fassbender's turn as David, an android with seething contempt for his human "masters." Scott revealed after 2017's Alien: Covenant - the direct sequel to Prometheus - that he wanted to transition the series from focusing on the Xenomorph to another "alien" lifeform with David, but the lukewarm response to the sequel seemingly killed those plans.
A third prequel from Scott appears to be off the cards, with Fede Álvarez directing a new movie for Hulu and an Alien TV show also being developed. That means the unanswered questions from Alien: Covenant - of which there are many - will likely never be answered. This includes the fate of Walter One, another android played by Fassbender who forms a bizarre connection with David when the Covenant crew lands on Planet 4. Walter (rightly) suspects the marooned android has twisted ambitions and rejects David's - who the series shouldn't abandon - attempts to recruit him in his quest to rid the universe of mankind. Walter is also seen winning a vicious battle between the two synthetics, but it's later revealed David won and took Walter's place, suggesting the latter was killed off-screen.
Alien: Covenant leaves this unresolved, but the fact David "killed" Walter previously by stabbing him in the neck, only for Walter to recover from the blow, suggests Walter has a more advanced design than David, which includes healing injuries quickly. This means it's possible David managed to deactivate Walter long enough to make his escape from the planet with the rest of the crew, but that the synthetic could have recovered sometime later. So the best-case scenario is that Michael Fassbender's - who some fans want to play Bond - Walter did recover but was left stranded on the planet.
David would likely want to make sure Walter couldn't interfere with his plans, but while he would have preferred to kill him to make sure, David didn't have much time to make his escape either. The Engineers were said to be a major part of a planned third Alien prequel too, so it's possible they would have landed on Planet 4 and recovered Walter, using him to lead them to David. Sadly, this is a dangling plot thread that's unlikely to remain that way.