So my guess about Mr. Gepard being the mastermind of everything was correct. Although some of the reasons turned out to be wrong. It's true that Mr. Gepard doesn't accept the engagement, but another reason is that he doesn't want his sister, Miss Serval, to depend on or be owned by someone else. Miss Serval was killed because Mr. Gepard felt that it was better to kill someone who had the possibility of being with someone else and leaving himself than to kill someone that she wanted. He was indeed the mastermind behind Miss Serval's death. Then, when the letter was found, a guest named Saeran remembered something and found another letter. The letter was about the misunderstanding mentioned previously, namely that the fact, sender of the letter (K), intended to send it to Mr. Gepard, but it was received by Miss Serval. Saeran became a victim and was killed because he knew too many things. The murderer behind Saeran's death was Mr. Sampo, Miss Serval's fiancé and the person who loved Mr. Gepard. The reason he was killed was because Saeran knew too much about things that could cause Mr. Gepard to be discovered if he was the mastermind, and Mr. Sampo wanted Mr. Gepard to keep safe. The possibility is, that Mr. Sampo knows that Mr. Gepard is the mastermind behind Miss Serval's death. So Mr. Sampo tries to make sure that Mr. Gepard is safe. However, it seems that Mr. Sampo and Mr. Gepard (perhaps) already thought that there was a possibility that they would be found out. Therefore, he had hidden the key (which was brought by Mr. Lyney) so that everyone could not escape from there. I don't know why—but it's possible that Mr. Sampo and someone with power over Mr. Lyney had a business or something. Because the powerless figure is not here, it is given to the figure's subordinates so that the key is kept properly. One would obey if commands were given in the name of one's master, right?