So Howard did not unlock the door to Charlie's room. Like I care, I know that Howard had a part in this!
After Bill left, Charlie locked himself inside a room (either with or without Howard). Howard had the masterkey in his possession until after his death, so he must have been the one who locked Charlie's door after Charlie was killed! Charlie let Howard in the room or they went into the room together in the beginning. After they were both in, Howard killed Charlie, left the room and locked it with the masterkey.
and if you dare saying that Howard did not kill Charlie, then
When Charlie opened the door for Howard, Lucy was waiting for the opportunity. She busted in the door and abused the fact that everyone was surprised, quickly stabbed Charlie to his death then using Howard's fear, she shouted at Howard and forced him to go in his room and hang himself.
The same thing applies if Howard and Lucy were working together, but with different motives. If Charlie didn't open the door at all while he was inside, then
Coward Howard and Scarely Charlie fled into the same room, they heard a knocking and Coward "dumbwaiter" Howard opened the door for Lucy, even if he didn't know it was her.
Please ignore my minor name extensions, you know what I wanted to say. xD