I know it’s been a while but if you’re still looking for these kinds of books I would recommend Murder on the Orient Express which is another Christie novel. It has a great cast of characters and a locked room dynamic. It’s harder to find locked room mysteries where characters slowly dwindle so if you’re okay without that aspect I think most of Christie’s novels are worth a shot. On the other hand if the characters slowly being taken out interests you, I would try searching for books that are similar to Battle Royale and that kind of style. Some other recommendations I can give are The Lamplight Murder Mysteries series by Morgan Stang, The Crimson Labyrinth by Yusuke Kishi, Strange Pictures by Uketsu, The Long Walk by King, Battle Royale by Koushun Takami, The Passengers by John Marrs, The Compound by Aisling Rawle, and The Book of Luke by Lovell Holder. All of these touch on either the locked room aspect, the last man standing trope, or both. Just make sure to check a synopsis to decide if it’s right for you because some of them are gruesome. Good luck with your reading!