Stanley or someone else visiting Stanley's room had previously opened the window (or propped the window open to keep it from self-closing). Stanley's killer used the stolen harpoon gun to be able to climb the outside wall to Stanley's room, and entered the room through the window. The killer then shot Stanley while inside the room using a gun, closed the window, hid inside a ceiling panel/the bed/the floor/the closet/some other space, and then left after me and Natalie discovered Stanley.
Stanley or someone else visiting Stanley's room had previously opened the window (or propped the window open to keep it from self-closing). Stanley's killer used the stolen harpoon gun to be able to climb the outside wall to Stanley's room, and entered the room through the window. The killer then shot Stanley while inside the room using the harpoon again, then kept the window open propped with either his body or an object, and then climbed down using Stanley's body as a post with the harpoon gun, releasing Stanley's body after landing (and thus, causing Stanley to drop). The object propping the window open was then either removed, shot off, destroyed, retrieved, or rendered useless at that point, allowing the window to self-close.
Stanley or someone else visiting Stanley's room had previously opened the window (or propped the window open to keep it from self-closing). Stanley at the time was standing on a/the table NEAR the window for some reason, allowing the killer to snipe him from below on the ground, killing him, and then the window prop was thus removed.
Stanley, due to height reasons, was standing on top of a table to open the window and prop it open. At that time, the killer was able to see him from below ground opening the window, which upon the window's opening, allowed the killer to snipe him from the outside, killing him, and then the window was closed shut.
Stanley or someone else visiting Stanley's room had previously opened the window (or propped the window open to keep it from self-closing). Stanley's killer used the stolen harpoon gun/or his bare hands to climb a nearby tree to get to the window's height, shot Stanley when he saw him through the window, somehow removed the prop holding the window open, and escaped. The window self-closed by itself later.