I buy that it's darker than most people realize however there is no second Marty as others have pointed out. I say it's darker because a YouTuber made me realize that in order for some scenes in the trilogy to be believable, Marty had to have died in a previous version. Doc has screwed with the timeline numerous times regardless of his stated viewpoint on the idea. Every time that doc saved Marty from death (jumping off of biffs tower, the tunnel chase scene, the hanging of Marty at the beginning of part 3.....) he couldn't have known that Marty would be there at that exact time without experiencing it in a previous timeline. Why did doc build the sniper rifle? He has never shown any interest in guns outside of when he shot at the libyans (which I'll point out, he was a terrible shot yet was able to shoot a rope in part 3).