I thought they made everything in this movie as ambiguous as possible.
He never dated the black girl. I actually thought the black girls affection for him was completely out of place and it was taking me out of the movie... until it's revealed he imagined all of their interactions, it was a welcome surprise.
In hindsight, he might not even have been given the gun.
They never really showed how he went from a triumphant leader standing on a car to a loonie locked in an asylum during a therapy session at the films end. Did any of it actually happen at all or did he make the whole thing up?
The strongest case for him making it up all along was that every single scene in the film is centred solely on him and never from anyone else's pov... except for one scene. The Wayne parents murder. Why would he make up that they were murdered and that someone else did it? After everything that happened in the movie he would definitely have taken credit for that himself as a revenge killing.