I realize in advance that this is probably too obvious to deserve a thread, but here it goes what I understood about the ending (spoilers below this point):
The corpse that we find in the closet is revealed not be Serena’s but actually its the protagonist corpse. Contrary to what the protagonist initially though, he didn’t kill Selena; Selena killed him. He was dead the whole time, and his ghost was amnesic roaming around the cabin trying to remember what happened. The two silluetes that we see through one of the windows are actually Serena and probably their mutual friend who was mentioned earlier in the game when we inspect the photo in the table.
I also presumed that Serena killed him accidently, during that heated argument that resulted in a crack in one of the windows. Or maybe it was on purpose and she and the mutual friend were having an affair. Is any of this correct? Or is this all open to interpretation?