Answer below:
It was Oswin. Original plan was for Roswell to use a sleepwalking Sal to kill him and then relied on the lack of recollection to have no one be able to piece it together. However, Oswin who was in the study intervened, and upon being unable to convince Roswell off his plan killed him in Sal's stead. Then, using Sal still sleepwalking to remove the axe from the scene, set up the book left in front of Roswell's body to assist Dave to potentially resonate with the event to circumvent it. Finally, he locked the door from the inside and then hid in the study, which the cast were unaware of existing, until they'd seen the body and moved on.