If Ronald is the murderer, why did he take the trouble to enter the room from the outside when he slept in the next room ?" "Surely you have not forgotten that the door was locked from inside?
Benson says Mr.Glenthorpe was in the habit of locking his door and sleeping with the key under the pillow.

Ronald no doubt first tried to enter the room by the door, but, finding it locked, climbed out of his window, and got into the room through the other window.

He dared not break open the door for fear of disturbing the inmate or alarming the house." "Then how do you account for the key being found in the outside of Mr.
Glenthorpe's door this morning ?" "Quite easily.

During the struggle or in the victim's death convulsions the bed-clothes were disarranged, and Ronald saw the key beneath the pillow.

Or he may have searched for it, as he knew he would need it before he could open the door and remove the body.