“Yes, I expect you’re right,” said Bill thoughtfully. “Unless he took the key in with him, and locked the door at once.”

“Exactly. But in that case you have to build up a new theory entirely.”

“You mean that it makes it seem more deliberate?”

“Yes; that, certainly. But it also seems to make Mark out an absolute idiot. Just suppose for a moment that, for urgent reasons which neither of you know anything about, he had wished to get rid of his brother. Would he have done it like that? Just killed him and then run away? Why, that’s practically suicide⁠—suicide whilst of unsound mind. No. If you really wanted to remove an undesirable brother, you would do it a little bit more cleverly than that. You’d begin by treating him as a friend, so as to avoid suspicion, and when you did kill him at last, you would try to make it look like an accident, or suicide, or the work of some other man. Wouldn’t you?”