A thought on the ending (spoilers)
When I first watched this, I was annoyed by the ending. I see from the message board that I was not alone. However, I've given it some more thought. While I'm still not crazy about it, I can appreciate it more in the context of the time period when the film was released.
One thing everyone has failed to mention is that with the censor board at the time, it was required to show anyone who committed a crime as getting caught or somehow having to pay for their crime.
Since they had planned to murder the blackmailer, it was expected that they would have to be caught in some way or another. Even though the blackmailer incurred an unexpected demise, it was still attempted murder, and the film code required that they pay for their crime.
Audiences at the time would have been expecting that kind of ending, and the dream ending solved the censor board problem in a quite unexpected way.