Okay, here is. As you see at the starting story we see the old man wearing a guard clothes but without security vast. He actually already died got killed by army. That why Soo-hyeon didn't interact with him.

Even you can see she (Soo-hyeon) didn't have a right mind state. She see something that not even there. The trauma that she had look severe. Not even her (Soo-hyeon) realize what she doing, she just thought the one who kills is Jae-yeon's her friend. She didn't get haunted by her friend, it's all her (Soo-hyeon) doing it by herself. And there's is no ghost actually it just illusion.

And the only part that I didn't get is who corpse that they found in toilet, blue shirt. The corpse also hold the army name tag.