This is a theory that I stumbled across that focuses on the Back to the Future series, and it says that there was a version of Marty that Doc killed. And well, it's kind of horrifying. Because as far as you know by just watching the movies, Doc never killed anyone. But it turns out he did.
Oh, and here's a quick warning: this theory might be a little bit scary because of the way I plan on wording it. If you love BTTF too much and you don't want it ruined by some guy on the internet, then you might want to stop.
Here's the theory...
In Back to the Future, Marty travels back to 1955 and asks the 1955 Doc to help him get back to 1955.
Doc is eventually successful in sending Marty back to 1985 using the lightning bolt from the clock tower, which was channeled into the Flux-Capacitor, sending the necessary 1.21 gigawatts required to time travel.
However, before Marty traveled to 1955, Doc was shot and killed by Libyans for stealing their plutonium. And he had planned on warning Doc about his future death in 1955. He is unsuccessful, so when he goes back to 1985, he sees what he believes is Doc getting shot by the Libyans and his past self traveling back to 1955. But it turned out that wasn't it at all.
Doc survives because he had taped back together a letter that Marty had written to him informing him of his death. And what about that Marty? The Marty we see traveling back in time at the end of the movie.
See, the thing is, this Marty that we see traveling back in time at the end of the movie is not the same Marty we saw time traveling at the beginning. The Marty we follow throughout the movies lived with the alcoholic Lorraine, nerdy George, and bully supervisor Biff. But when Marty hopped into the time machine and inadvertently traveled to 1955, he changed the past. He interfered with his parents' first meeting and though he got them back together, he did change the future.
Lorraine becomes happier and healthier, George is now a successful writer, and Biff is now kind to everyone. And this is the family that the Marty we see above grew up with. He even had the Toyota Statler 4x4 that Marty wanted in the beginning.
So, if Marty's entire family changed, then Marty might not have been the kind of guy who would hang out with a scientist. So, in the new timeline, Doc knew that he had to track down Marty and build a time machine to send Marty back in time because he knew that on October 26, 1985, the Marty he met in 1955 would show up and if two Martys met, it could destroy the space-time continuum.
And we see this happen, because at the end of the movie, we see the "new Marty" travel in time. But where was he sent in time?
I mean, he couldn't have been sent back to 1955 because there's already a Marty there. And he couldn't have been sent far into the future because he could potentially do something dangerous that could destroy the universe. So as Doc is so smart, he must have sent Marty WAY back in time. So far back that the Earth was still molten lava back then. He sent Marty to a time where Marty would drown in lava and therefore, he sent Marty to his death.
So, at the end of the movie, when we see what we think is just a different Marty time traveling to 1955, we're actually seeing a different Marty time traveling to his death, making Marty a martyr.
You may have thought this theory wasn't all that scary. Okay. So let me rephrase the story of Back to the Future.
"In 1955, scientist Doctor Emmett L. Brown has a revelation. He comes up with Flux-Capacitor, an ideal component in making a time machine work. Later that day, a 17-year old named Marty McFly arrives at his door from the year 1985. Doc knows that he made the time machine work, testing it on October 26, 1985. Doc sends Marty back to 1985 and he makes the time machine out of a DeLorean DMC-12. He tracks down the Marty born in 1985 and tricks him into time traveling to his death. Doc tells him that if he doesn't, he would destroy the space-time continuum, and he says that his parents hated him, were better off without him, and that Jennifer wanted to date someone else and was planning on breaking up with him. Extremely depressed, Marty jumps into the time machine and accelerates to 88 mph, sending him to before the Earth was formed. Marty drowns in lava, killing him."
Yeah. Yeah, that's kinda sick.