Okay, first, Fred is murdered by a person hiding in his locked bathroom, the curtains are then shut, and the person returns to the bathroom, locking it. Then Bill, opens his window and leaves, attempting to get Freds attention Via knocking on window (Fred is dead, yet curtains are shut so he is unaware) Bill gives up and goes to Bobs window, where he gets bobs attention, he explains his bathroom is locked, and he needs to use the bathroom (For any reason, shower bath toilet (ect ect). Meanwhile the murderer in Bills room leaves the bathroom and follows Bill. Bob lets bill into the room, as Bill closes the bathroom door, the murderer sneaks in, murders Bob, then he opens the bathroom door and murders Bill, if the door is locked, he waits for Bill to finish using the restroom, regardless, Bill is murdered in Bobs room. The murderer then Unlocks Bobs room, goes back to Bills room through the windows, and opens Bills room. He then transports Bills body back into his room. He then returns to Bobs room, locks the door and leaves through the window. He enters Bobs room through the window, locks both the window and the door, then hides in the bathroom. Freds murderer then opens the curtains.
After the large amount of noise generated by the smashing of the window alerts the murderer he sneaks out after you, going around the opposite way around the house, sneaks into Bills window through the window (At the time, you are breaking the second window) The murderer then locks the window behind him, and retreats into the bathroom where he locks the door. After you run back into the front room in panic, both murderers leave through the broken windows you created.