tell the doctor about the assassination plot just before he died?
Did he do that on purpose or did the doctor just happen to be there? Didn't the movie have a scene about how he confused the american couple with that bad british couple or something??
Doctor McKenna just happened to be there, and Bernard had met him the day before and learned who he was. Presumably he told McKenna what he knew because he felt he could trust him enough to tell the authorities. In the circumstances, about to die, Bernard had to tell someone what he knew. If McKenna hadn't been there I suppose no one would have learned his secret. (There was no scene in the film about Bernard confusing the McKennas with the Brits.)
Like a lot of Hitchcock this point is a bit stretched. We never know whether Bernard knew about the British couple, why he's running from an assassin but with two cops chasing him (the killer), why he's dressed as an Arab. It all looks mysterious but is never explained, more smoke than fire. One also wonders that if the police were chasing the killer chasing Bernard -- even before he stabs him -- wouldn't they also have known who Bernard was and what his secret was (or at least something about it)?
More to the point, we never learn why Bernard tries to find out so much about McKenna the day before. Was he setting him up as a decoy? If so, why? He certainly didn't know he would be killed the next day, let alone that in his last moments he'd just happen to stumble across McKenna in the Marrakesh bazaar. Here again, yet another plot development that if looked at logically really doesn't make much sense, and very typical of Hitchcock.