Obviously, it is not a great idea to ride alone at night in many areas. If you are ever close enough to a human attacker to get more pepper spray on him than you get on yourself, you have gotten way too close. You don't want to be close enough to have to use that stuff.
A bike and rider weighing 150 pounds or 200 pounds and moving twenty miles an hour is NOT something a person wants to be in front of. Bright lights will tell someone you are coming, and putting on speed tells them you are not stopping. Ride near the center of the street, have bright lights, ride fast, and don't slow down when someone says "hey, you looking for something..." Don't speak...just keep accelerating.
If you ride fast in the dark, people can not be sure who YOU are. When I am wearing dark shorts and a white helmet, plus the bright lights on my bike, I sometimes hear people on the sidewalks saying stuff like "Put that stuff away, that guy is a cop"...the police patrol my town on bikes, wearing shorts, white helmets, and riding bikes with bright lights.