The first letters of each chapter spell out a sentence when you line them up. Nicole Is Dead.
At first, I was annoyed that, after the initially realistic handling of sound in a vacuum in Dead Space, some external objects suddenly started making sound... until a little later, when I realized this was happening for things that vibrated the surface you were standing on, meaning that it was truly being handled realistically, rather than just making everything silent except for player-sourced sounds. - Nezumi
To add to this, if you are hit by the pressure wave of an explosion, the air in your suit reacts to it, making a muffled boom sound. Again, this is realistic as you are RIGHT next to the explosion so of course it will affect you.
How can Isaac sever limbs, crush skulls and cut through bone with a simple curbstomp? Simple - he's wearing magnetic boots. As he's about to stomp, he activates them momentarily, pulling his foot down hard enough to facilitate Ludicrous Gibs.
Many people complain that the strategies for dealing with dead bodies is obvious and simple (shooting necromorphs that appear to be dead; stomping dead bodies to prevent infectors from using them), but that is only because they are only watching the situation. Isaac must shoot any dead enemy he finds on the chance they are faking and desecrate human remains because they might attack him? While dealing with the necromorphs is simple because they are nowhere near human, he has to stomp through dead bodies with his own foot, feeling their snapping bones and muscles. There is a reason why he starts frantically cursing when he stomps in Dead Space 2.
Ever wonder how the hell it makes sense that by blowing off all the limbs with the mining tools, you kill the necromorph? Perhaps it's explainable by the fact that they're all telepathically controlled by the Hivemind. When it detects that the necromorph no longer has enough limbs to be functional, it could just be withdrawing its control, thus 'killing' the necromorph.
With the revelation in the sequels that the Necromorphs seek to spread the infection and are capable of remaining dormant for hundreds of years, the theory that the Necromorph's are simply rendered dormant by having their limbs blown off, makes a lot of sense. Being unable to spread the infection further, they switch off and become biomass waiting for the time of Convergence, when they will join with other necromorphs to become a Brethren Moon.
Or a necromorph could be considered a parasitic life-form that inhabits the entire body (when you cut off limbs, what looks like tentacles wriggle around for a few seconds before retracting inwards). Shooting the torso doesn't work because the creature inside is composed of tentacles animating the body with no core inside, and there's very little it needs to control in the torso, but cutting off the limbs damages the organism inside severely. Damage enough, and the organism inside dies, and the corpse it controls goes inert.
The above theories don't prove why blowing their freakin' heads off just makes them angry.
How about this for a theory. The Necromorphs have a distributed nervous system, as opposed to our centralized nervous system. Shooting off limbs makes it harder for the Necromorph to think. As for blowing off their heads You sick perverted bastard, the head just houses most of the senses, but not all of them.
HELL, in Dead Space 2, you see how a slasher is made first hand, one of the first things destroyed by the transformation is THE EYES. What the fuck?
While some Necromorphs do fair less well without heads than other (slashers in particular will flail about attacking in your general direction) their severance from the main body hold true with the logic that the chest is an equally useless body part. Whatever stringy wiggly organism that is in and defines the Necromorph, isn't harmed much when it's tips in the head of the subject are blown off - especially in comparison to how long the strands in the arms, legs, or tail must be. Therefore severing the head and shooting the chest must do damage, and it does, but not much.
If the tendrils are like nerve cells or fungi mycorrhiza and they are in the limbs, then that explains why the head isn't vital to keeping the necromorph alive and why cutting off the limbs does kill them. You're cutting its brain apart by going for the limbs.
The fall of the USM Valor is generally regarded as Fridge Logic at best. However, a number of considerations make it far more plausible than it initially seems. First, there is no indication that the single Slasher inside the escape pod was the only necromorph to travel on that pod; any number of them could have been clinging to the exterior. Second, even a basic Slasher takes a long time to kill if you are shooting it in the center of mass, as soldiers are trained to do, and even longer if your weapon is a pulse rifle with few or no upgrades, as grunts are likely to get; if the ship has any fully upgraded rifles, they are presumably restricted to the higher ranks, who wouldn't be in charge of opening an escape pod. Third, at least one form of necromorph must be capable of spontaneously generating, without the presence of an Infector; otherwise, how could any of them form at all? If the necromorph virus is capable of spontaneously reviving Marine corpses, that single Slasher becomes a Slasher and an unspecified number of Stasis Slashers. Fourth, just try hitting a Stasis Slasher with your own Stasis module, and shudder as you realize that even an upgraded module (which, again, grunts wouldn't likely have) doesn't mean anything if you can't hit the target in the first place. Fifth, apart from all the physical disadvantages facing the crew of the Valor, they would have to overcome the psychological resistance to killing their former comrades. Sixth, the biggest battle on the ship occurs in the barracks; presumably, at least some of these soldiers died in their sleep. Finally, given that necromorph corpses disappear once Isaac leaves the room they are in, it's entirely possible that "dead" necromorphs are being reabsorbed into the biomass, and springing up again later on. Maybe the Marines did kill the Slasher (and any other necromorophs that may have traveled with it) - but they made the mistake of thinking it would stay dead. Taken together, these points turn what seems like the biggest headshaker in the game into its most horrifying sequence.
Also, Fridge Brilliance really sets in if you find the Unitologist graffiti on the Valor. One or more members of the crew may have been devout Unitologists like Mercer, who misled or outright sabotaged their own crew in a deliberate attempt to "ascend."
Marker insanity is also possible, but all of that doesn't explain why a crew who knew that there was a possible infection and was sent to contain it wasn't capable of dealing with it. Why didn't they have biohazard training? Also, the crew dies incredibly fast. It would still take time to make all those necromorphs without an infector. It took days for the colony to be overrun. It took the Valor less than 10 minutes.
After the Valor hits the Ishimura, Hammond is able to communicate with you again, saying that somebody was jamming his comms. When we later find out that Kendra was the mole and not Hammond, it seems likely that she was the one jamming his signal to cut him off.
You ever wonder where all that flesh mush that starts to cover the entire deck comes from? Obviously it comes from dead biomass, but that also explains what happens to the corpse you leave behind. They become part of The Corruption, the fleshy growths you find covering the walls and floors in the later chapters. When Isaac kills everyone and then leaves, the dead bodies and limbs are rounded up and merged with the growths, giving it additional biomass to spread. Or they could even go into the creation of more advanced forms, such as the brutes you find being created from body parts left around. Even dead bodies don't have to be touched by an infector to be claimed by the infestation, like how the Guardians were just people killed on the growth.
70% of dust is dead skin. The growth probably starts from the dead skin cells that naturally fall from people and accumulates over time. Then we add corpses and bodily fluids (as mentioned above). Finally add oxygen and suitable conditions for growth. The result? Meat walls.
Nicole's message involves her killing herself on camera after saying goodbye to Issac. Which sounds really messed up, and kind of a cruel thing to do to you SO... unless you very definitely don't want him to come looking for you, on account of horrible alien monsters killing everyone on your ship.
Now, you'd think there would be no subtle horror to Dead Space, seeing as how it's pretty frikkin' obvious that it's a Survival Horror game, but there is at least one point where Fridge Horror rears it's ugly head. Let's take the second chapter. You go to the hub where you start your missions. In the corner between the two mission entryways, you find an ammo pack. So far so good, right? Towards the end of the chapter, you return to the hub and the corner is no longer unoccupied... There are a number of body bags (read: handy Oscar Meyer Zip-Loc snack packs) there, now. They're gone again in Chapter 5. EEP! This means that the necromorphs are watching your every move. They know where you are and can come get you whenever they're good and darn ready, Senor Lunchmeat.
Or, mayhap, an even worse thing may have come up in which you realise that you took the ammo from the storage locker and then other soldiers, who needed the ammo, died because you decided to take even more ammo. You heartless bastard you.
Now, depending on how fast the necromorphs spread, the whole crew could have been killed off in minutes. Now just imagine something like THAT spreading to earth.
Okay, and how about something else? At some point of the game, near the end, Isaac meets his girl, he must protect her from death (asking her where she was hiding in the first place never occurred to him). If she dies, so does him. At the end of the game Isaac finds out that his girlfriend was dead long before he even got to Ishimura. And all this time the Marker was feeding him illusions, in order to be brought back to the planet surface. Okay so how PC should we see his possible death while failing to save her that time? Or better yet, what could possibly be the cause for Dead Space 2?
Think about it. Isaac could STILL be feeling the effects of the marker as he goes to the ship... what if the Necromorphs AREN'T necromorphs...
Or alternatively, think of Isaac's girlfriend's message. He must have played the entire message on board the Kellion, as Kendra asks how many times Isaac will watch it, and knows how it ends later in the game. That means he knew she killed herself and yet during the entire game, he thought she was still alive and was looking for her. That means the marker had nothing to do with it, Isaac had gone mad before he ever arrived!
I doubt that anyone would have allowed Isaac on the mission if he would be so obviously unstable. Remember how Hammond talks to him about Nicole still being potentially alive and that he will try help him find her? It's more likely Isaac cut the transmission before the bit were she kills herself, fooling himself into thinking she is still alive. Only Kendra could have known since she was given intelligence on every person on the mission.
Or Kendra edited the transmission herself to keep Issac in the dark.
The space tow truck company wouldn't allow someone who's spouse just died onto a mission to fix a radio? They're not soldiers or secret agents. And he doesn't have to be "writing things in his own blood," crazy, just "I've blocked out the memory of the end of the video and refuse to face her death," crazy.
Didn't matter, they were all going to die any ways for Kendra's mission. Maybe he was all they had on hand for the repairs as well.
Talking about Nicole. Look at her screen flashing on board of the Valor. Her face briefly turns into a skull for a split second. Now, why would the marker do something like that if it wants Isaac to think that she is still alive? The answer, it doesn't. It's Isaac own mind fighting against the marker influence trying to remind him that she is dead.
The Virus animates dead tissue. This actually wouldn't stop it working on the living, since hair, fingernails and the outer layers of skin cells are all dead tissue; so is the majority of household dust. This realization can create hideous mental imagery. One of the logs obtained after beating the game supports this. A strange growth appeared in the ventilation ducts of the research facility that discovered the necromorphs.
In the first novel, Martyr, one scientist accidentally injects himself with a Necromorph tissue culture; he begins to change immediately and eventually turns into an Infector. That means it can affect the top couple layers of your skin, your hair and your nails, but the pathogen's not airborne. As to why the Corruption starts growing, perhaps the Infector drips it's viral culture everywhere, starting it growing in the dust.
Related to the Nicole fridge brilliance up there. If she didn't want Isaac to come for her, then why didn't she just tell him what was happening? In fact, if you are capable of sending out messages like that, then why can't you inform people that there's a bio-infestation and have them pass on that information? How did she send it if the communication satellites were damaged? Was Hammond's crew closer to the system? Why did Earth Gov put some warning beacon down after the incident? Seriously, the lack of any signal causes no less than four more incidents. Hammond's crew from the Kellion boards and gets trapped, the Valor comes in and gets slaughtered, "Magpie" salvage ships come in and find Marker pieces (it doesn't end well), and Nolan's ship comes in and also gets screwed over by Marker pieces. The first infection is tragic, the fifth one is just pathetic.
On the Valor, you find that the grav-plating is broken in places. There's a soldier pinned to the ceiling and he's still alive. Okay, one: how is he still alive where ever over instance of something biological thrown upwards is gibbed, and two: why can't you save him?
Another one from the USG Valor. Ok, so Isaac is just an engineer wearing a welding suit and using power tools for weapons, and he slaughters at least 200 Necromorphs over the course of the first game all by himself, including some giant ones. So... exactly how did an entire ship full of trained soldiers with real armor and real weapons get completely overrun with what was implied to be one Necromorph who stowed away in an escape pod? Nobody on the ship was able to kill just the one? The slasher we see in the video call from the soldier wasn't even an enhanced slasher, it was just a regular old pink one that bites the dust with about 5 shots from a plasma cutter. Nobody was able to stop it before it slaughtered the flight crew?
There are several instances where a door is locked until someone is killed. The guy knocking on the door to the room with the plasma cutter, the scientist killed by the Lurker, Hammond, and Jacob Temple. Why does the door open after they're dead and why can't you just break the glass/smash down the door with your tools made for cutting metal and pulverizing rock? An elite Slasher (made of Captain Mathius' corpse) easily breaks through glass similar to that and some of you weapons are more powerful than their claws.