Right, so Isaac's walking down Late To the Party Blvd. and all of a sudden, a Necromorph jumps the flying frig out of nowhere, lopping his head off in a single swing. There is nothing to do to stop this, it's literally scripted.
Eventually, Isaac comes to... but he can't move his body at will. It's almost like someone else is controlling his body. That is, until he sees his own decapitated corpse, and is fairly sure that his soul has left his body after death somehow. He then comes into contact with someone else in his thoughts, when he slowly comes to realize that he's talking with the tortured psyche of a Necromorph victim, who is fully aware of what's happening to it. The two converse, and Isaac comes to realize just how nightmarish the Necromorphs are, and that this could have happened to him if he was still alive.
Isaac's conscious is then absorbed by the apolegetic Necromorph victim to a world of eternal pain as the victim's company. At this point, he wakes up, crumpled into a heap on Late To The Party Blvd., and realizes that the Marker was screwing with him... but the Necromorphs might still be in this pain, and if he doesn't get out while the getting's good, he may very well become one.
Aright, so I think that was kinda creepy, but that's nothing on this one... I think, your mileage may vary.
Isaac's going about his business, killing, stealing from corpses, etc. when all of a sudden, he hears a whisper. He calls out, and looks about, and finds no-one. So he goes back to his survival, writing it off as his mind cracking. But then...
They took my face!
So, naturally, Isaac's pretty spooked by this point. He continues onwards, though, and...
I KILLED HER...! WHY!? WHY!?!??! WHAT AM I!??!!!??
Forging onwards, trying to block it out the speech, Isaac comes across something interesting...
A Necromorphic corpse, its face heavily chewed, and a female corpse nearby. Also nearby, the skinned face of the Necromorphic corpse, found stuck on the wall by a peice of the Corruption with a horrified look on its face.
WHAT AM I!??!!??!!??!?!?!?
Isaac turns around, weapon at the ready...
Both corpses are gone, and the face has dissapeared with them.
So, yeah, I thought that was scary.
Alright, now the Necromorph.
Basically, it whispers to the player to act as something to make the player doubt Isaac's sanity. The game will screw with the player at times, as the Necromorph can often be found around the next corner, as it can't run in vents due to being just a mouth and a few vocal cords hanging from a wall by its own congealed and corrupted blood. But sometimes there isn't one... and you may find yourself qustioning your own sanity. If you get too close to melee it and save ammo, it will scream and bite Isaac, alerting all Necromorphs in the area and telling them all to go to your location, aside from the ones playing dead, as well as taking off some health.
So, yeah. Creepy.
Now some people may be saying "This all has to do with giving the player and insight into Necromorphs." And that's because I got it all from a single comment, which I can't remember because I made all these ideas up on the spot and didn't think it was worth it to hold in my mind.