generally it is unethical. There may be some exceptions, for example the author is already deceased and it is impossible to ask their consent. Other case may be when data in the draft are of the vital importance to the public, but the author withholds for ulterior motives.
Regarding the criminality it may differ depending of the jurisdiction, but I'd say it's violation of copyrights.
Evidently this is an immoral fact in any part of the world, if the victim can prove that he was the subject of a crime, it can become a criminal act and it can be taken to court. From the moral point of view is something unspeakable, what should exist among scientists is collaboration, exchange of ideas, solidarity, teamwork, no more than that. Scientists, without renouncing the class character of science, must be above national borders and the petty interests that hide behind the games that move the springs of scientific research on the planet. We work for humanity, for men all over the planet, not to profit from the results obtained, much less through an act as immoral as using an unpublished manuscript. In my country there are offices that protect the authors, but if the work was not legally protected what remains is to demonstrate in a court that was the object of a plagiarism, rather of a theft and treat the fact as a common crime.