Well homeless people do die in parisian streets in the cold winter days but beyond that I really doubt that you'd be very likely to get killed sleeping on a bench in France (or frankly in most civilized places in the world, why would people randomly kill people in the street?). You'd probably end up with a cold though, parisian nights are seldom hot enough to comfortably sleep without any protection. Regardless I wouldn't exactly advise doing it, theft is definitely possible.
