"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?"
"You mean you'd give yourself a bit of a run for your money?"
"Yes, that's what I mean. If you were going to do it deliberately, that is to say and lock yourself in before you began."
Cayley had been silent, apparently thinking over this new idea. With his eyes still on the ground, he said now: "I hold to my opinion that it was purely accidental, and that Mark lost his head and ran away."
"But what about the key?" asked Bill.
"We don't know yet that the keys were outside.
