The Mystery of Mr. X

The Mystery of Mr. X is a 1934 American PreCode crime film starring Robert Montgomery as a jewel thief who gets mixed up in a series of murders in 19th century London. It is based on the novel X v. Rex by Philip MacDonald under the pen name Martin Porlock.

Nick decides to give up his life of crime for Jane. He mails back the jewel. However, when Joe warns him that Hutchinson has been picked up for questioning, he realizes that it is only a matter of time before his associate gives him up. Nick discovers that the locations of the murders form an X, which provides him with the site of the next crime. He disguises himself as a policeman and flushes the real killer out. After a struggle, Mr. X is fatally injured, but before he dies, he boasts to Sir Herbert how he close he came to fulfilling his goal of one murder for each of theyears he spent in prison.

Source: Wikipedia