I don't remember how the dream began. Like many of my dreams it took place in one of my childhood homes, except that the wall dividing the two bedrooms at the end of the hall is gone so that it is one big room. A co-worker of mine (I'll call him K) is intentionally scaring people. But not in a leap-around-corners kind of way. More in an extremely-creepy-I'm-intending-to-murder-you way.

I'm watching him from the end of the hall. He's wearing ancient Chinese armor. It's all brown and black, the front and back are dark brown tiles linked together, with black plating underneath. He's wearing kind of a conical helmet with a face on the front, and a face on the back. The armor is designed in such a way that it's very difficult to tell which direction he is actually facing in. K is moving around the two people at the other end of the hall, twisting and sliding and bending like a contortionist.
This is about when I become aware it's a dream. I go to the other end of the hall, though he really scares me. I see that he's pulling a small bronze knife with a triangular blade from inside his armor. I can see another one. I make the choice to steal it as he's twisting around the man at the end of the hall. I believe that this is my only chance to stop the horror of K. So I grab the knife from inside his armor and stab him in the neck. The blade goes in, but he is bloodless. He takes the helmet off and removes the knife from his neck. It slides out like his skin is a sheathe. K slides the knife back into his neck and pulls it out several times, grinning at the three of us. I'm horrified.

The dream skips scenes. I'm standing at the start of the hallway again. I am very tired and I can see my bed. I'm really looking forward to sleeping. It has a tarnished brass frame and stark white sheets. That's when I realize K is laying in the exact middle of the bed, his presence deeply dismays me, though now he's just wearing a white t-shirt. The armor is gone. I assume he's not playing the creepy guy anymore, and see that as a role, a costume. I'm afraid, but I'm tired and I just want to rest. I walk to the bed and pull the sheet back like I'm going to lay down, but then I just stare at him. I can see the bloodless slit in his neck where he was keeping the knife. It really unsettles me. I go into the black doorway across from the bed, knowing it's my parent's bedroom. I realize the couple inside are not my parents, and I don't really know what I expect them to do about my problem. But, like a frightened child (that's even how I identified this action in the dream), I wake the man by lightly touching his foot.
"I don't know where I'm supposed to sleep....K is in my bed."
"It's alright, don't worry about it." The man says.
"But I'm afraid."
The man doesn't respond, I know he's gone back to sleep. I return to my bed. K rolls over and sits up. I just look at him warily.
"It's okay if you lay down." he says.
Reluctantly, I get under the sheets. I'm still afraid. He tries to reassure me that he's not going to kill me, but as he says it he pulls the bronze knife from under one of the pillows and casually slips it into the slit in his neck. He grins. I'm not reassured at all...

I honestly don't understand. As a person, K doesn't scare me. We're even work friends. On top of that I very rarely have nightmares, so I don't know why I had this one. Nothing has happened at work with K to trigger it.