In the climactic gathering of the suspects, everything becomes crystal clear except for one thing- no reason was offered as to why Crazy Mary was murdered. Peter Berton yes, but Crazy Mary no.
"I take pleasure in great beauty" - James Bond
Maybe because Dr. Clayworth felt that since she was really Peter's mother, she wouldn't rest until she brought his killer to light, or maybe uncover the monkey shines with the paintings? He was very surprised to learn she was Peter's mother.
"Strange women lying in ponds distributing swords is no basis for a system of government."
-Dennis