scary (adj): frightening; causing fear.
If there's one word to describe this theory from the well-known time travel series, Back to the Future, it's scary. If you're easily scared or upset, you might want to stop reading here. If you want to take a risk and you don't think this theory will be THAT scary, you can keep reading.
At the end of the original movie, Marty travels from 1955 to 1985 using the lightning from the clock tower. He then races to the Lone Pine Mall and there, he sees Doc getting shot by Libyans and himself traveling with the DeLorean. But where did this "other Marty" go? There's no way the Marty we see travel back went to 1955, because Doc already knows there's a Marty there. If two Martys meet, it could destroy the space-time continuum.
Due to Marty time traveling in Back to the Future, the Doc in 1955 knows that in 1985, Marty will travel back to 1955. Marty even has a video to prove it. And after Marty goes back, Doc knows that he will have to befriend a teenager named Marty McFly and he knows he'll have to build a DeLorean time machine.
When the Doc we see at the very end of Back to the Future sends Marty back in time, it's actually to send him out of the present so that the Marty who Doc knows will get back on that exact day and time can come back without meeting his other self. In fact, if Doc was smart enough, he would give the DeLorean only one shot of plutonium and send him to the birth of Christ (which Doc demonstrated), or maybe, Doc could've sent this other Marty to a time with no humans to help him get back to the present.
Maybe there's something else to it, too.
When we see Marty travel back in time, and Doc getting shot by the Libyans, they act the same way. This could be because we see the 1955 Doc watching the tape of 1985 over and over again. Doc could have trained or coached the new Marty to react that way. ("NOOOOOOOOOO!!!! BASTAAAARRRRDDDSSS!!!")
In fact, after 1955 Doc sends Marty back, he could have tracked down the new Marty, befriended him, gained his trust, and makes the new Marty willingly time travel to his death to save the space-time continuum. Thus, making Marty a Martyr...or whatever that is.
That wasn't so bad, was it? It's just a theory about how one of your favorite mad scientists gained the trust of and murdered a young teenage boy. Nothing too terrible, right?