When Agatha Christie wrote plays beased on her novels, she sometimes changed who the killer was -- because it was as easy to do as you observe. (I nonetheless have a tremendous weakness for her novels, and have recently reread 15 (!) of them, with no end in sight.)

I of course also love the Sayers books. Another classic British mystery I admire tremendously is Green for Danger by Christiana Brand -- great setting (rural hospital during WWII), and I liked all the characters so much I was quite distressed that one of them would have to turn out to be the killer.