system
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human
[Dead Space] Why do the Necromorphs attack Isaac?

I just finished the first Dead Space and I have to say that I do not understand a lot of things, for example how Nicole could interact with consoles if she was just a hallucination. Also how Isaac was so stupid to think it was the real one. And, If she could control the consoles etc, why didn't she bring the Marker back to the planet herself?

But most important: The Necromorphs are all controlled by the Hive Mind. And as I understand it, Nicole was the Marker and Nicole tried to bring the Marker back to the Hive Mind, where it belonged. To make the Marker and the Hive Mind whole again. But why would the Hive Mind then attack Isaac? He did everything the Marker and the Hive Mind wanted? And why did the Hive Mind seemingly attack Nicole?

If the Hive Mind doesn't want the Marker to come back why the whole Making-whole-again thing?

I just cannot wrap my head around that. Could someone please explain this to me?
gpt
As for the consoles, its possible it was just another crew member that Issac was hallucinating as Nicole. Or it was time to an automatic cycle once someone used the trolley.

The ultimate goal of the Markers is to achieve convergence. That is, to surround themselves with so many Necromorphs (modified human DNA) that it triggers a special event, where the mass of flesh is converted into a giant spehere (called Bretheren Moons) and the Marker is absorbed, creating a being that can travel the stars and continue its mission to to absorb all life in the galaxy. While the Ishimura had a lot of biomass on it, it wasn't enough to achieve convergence. To achieve that, it needed to go back to the Colony, which is had already managed to transform (hence the giant monster at the end).

As for why the Necromorphs attack Issac, its not really sure. Perhaps to expedite his mission by keeping him jumpy and on his toes, so he won't question the Marker's orders? Maybe the Markers don't have individual control over individual Necromorphs until they reach Bretheren Moon status. Attack "Nicole" is obviously just a ploy to throw off suspicion, as it needed Issac to trust her so it could manipulate him.