Marseille is (and has been for years) listed as one of the most dangerous cities of Europe.
A few years ago, too many people showed up for the last bus from NYC to Montreal. My friend and I slept on the floor in Port Authority (NYC) to catch another bus the next morning. Nothing happened to us... Doesn't mean sleeping on the floor overthere is a good idea. Doesn't change the fact that Port Authority is one of the most unsafe spots of Manhattan.
All I'm saying is, yes, you have to be careful everywhere, but in some places, you have to take more precautions... And that according to many reports + my friends experience, Marseille is one of them.
Of course, you are more than free to ignore my suggestion...