Harune wrote...
ok, took me a while...
this will be my last attempt for today.
Melody is the killer.
#1: She has made everyone believe that Karen had died, by using the bottle of blood, then she hide Karen, that was still sleeping, in the bathroom
#2: After the light goes off she pushed Johnny out of the window, after that she goes to the bathroom. Nina hurt the chair and activate the secret switch, so the light came back.
#3: She killed Karen during her sleep and she hide the corpse in the bathroom. Then she left from the bathroom window, after broke it.
#4: She returned inside and she pushed Nina and William down the stairs, Nina will die slowly, after everything will end.
#5: She killed William in the kitchen. After that she jumped out from the same window where she pushed Johnny, dying.
Well, there's no secret switch there... but your deductions are close to the truth!
Well done Harune!
This is what really happened
Start
-Melody drugged the drinks
-this made Karen sleepy, and made William to void
#1: William went out, Nina and Johnny were busy playing racing game.
Nina went upstairs, took Karen into the bathtub then put a pool of blood near the bathroom.
She rejoined in the group, Nina screamed, and Melody herself "checked" the bathroom to lie that no one is there
#2: Fear has started to roam around, the group stayed into the kitchen.
Since Johhny and herself are close to the window, she made a distraction in order not to be seen by anyone while turning off the lights. She had an extra switch, flipping it twice in order to create an illusion that there's no power.
Melody then pushed Johnny into the window, also pushing the chair towards Nina to create a sound that can hide the real switch's sounds
Unfortunately, she had lost hold of the switch while pushing the chair.
#3: Since they weren't moving at any place, she insisted to go to the bathroom to kill Karen who will wake soon
Nina immediately notice this, and went to Karen's room which is close to the bathroom.
Nina spectated whoever's going out of the window, and saw Melody.
Melody went out of the window, and smashed it using one hand.
Nina and William went there to check, but Melody is gone.
#4: After William and Nina went out of the bathroom, and proceeded to go into the kitchen. Melody went inside via the window and sneakily followed them into the staircase. Pushed them hard, but William survived the fall
#5: William ran into the kitchen, and saw the fake switch lying on the floor and knew who's the culprit
Melody then went into his back and said "Took you long enough!" and killed him
Knowing that police will come sooner, she immediately go back to the bathroom and jumped on the window.
CASE SOLVED