This is the old west afterall and a good old shootout is how they settled things ...
It isn't actually.
Here's something else.
When Alternate 1985 Marty travels back to 1955 he should crash into himself.
[Our] Marty arrives in the alternate 1985 -not the dystopic one- he goes to the mall. There's some important things to note here. He
watches the events from earlier in the movie take place. We
know the place he's now in is changed by it being the "Lone Pine Mall" now and what we later see of his home life. This means this 1985's past is based on
his intervention. That means in
this 1985 Maty's past
our Marty was there.
Our DeLorean was there, etc. This means when this Marty goes back in time he would
crash into our Marty. By this movie's own time-travel logic.
This isn't a "pre-destination paradox." This movie's time-travel isn't working like that. If "this" Marty hadn't traveled back in time his past wouldn't have been effected. It had already occured, his past and present isn't dependant on him traveling back in time.
So when he arrives in 1955 he should crash into Marty, actuall appear right on top of him. This movie practices a non-linear time-travel. To "prove" this we'll say it like this. Say that instead of Marty-A going back in time in that exact spot he goes back in a slightly different spot. With this movie's time-travel logic he should be able to go back in time and
see himself and what really happened to bring his parents together.
So when we see Marty travels back in time he's either got to crash into himself or, maybe, while in the "continuim" between the time periods he, or "our Marty", will be displaced into nothingness.