When the Libyans came for Dr. Scott, Marty had no choice but to jump in the DeLorean, open it up to 88mph and thrust himself back, into Hill Valley’s past, seeking refuge from the dangers of the present in the relative safety of history.
What he hadn’t known, in that moment, was that he wasn’t alone in the car…
Musca Domestica, one of the most common insects in the world, and not the sort of thing anyone, in the heat of the moment, running for his life from terrorists who’d inexplicably brought along a rocket launcher, would notice inside the car with him, but there it was, tagging along without his knowledge, for the ride and its horrible aftermath.
Because the time machine, you see, had only been built for one, and Doc Brown had had no idea what, if anything, would happen if two living creatures were shunted back in time simultaneously, if he’d known he’d have taken some provision, that such a thing might be prevented ever happening.
Of course, at that point Marty would only have been shot by a Libyan terrorist with a rocket launcher, so there’s every chance that even if he had been warned of what was to come he’d still have made the choice he did, assuming that with life comes hope, however slim, and as such assuming that more life, even a little bit, would be the preferable option.
And, at first, that seemed to be the case.
Although he had changed, had been changed, by what had happen to him, at first the results of this change were universally positive. He found himself stronger, faster, his senses heightened by what he’d been through, in ways he couldn’t begin, even with the assistance of a much younger Doc Brown, to explain. He’d become a superman and, stretching his now much more powerful legs in a version of Hill Valley 30 years previous to the one he’d known, he reveled in the power he’d been granted.
Biff something, the bully who’d so tormented his father in their time together at school, was found dead, his arm broken in two places, his neck snapped similarly. And you don’t even WANT to know what he got up to with his then high school aged future mother…
But things inevitably went wrong, horribly wrong, as they had always had to do. Primate and inectoid DNA, after all, are not designed to blend, and by the time Marty dragged his tumored, bloated, disfigured wreck of what once had been a body, now completely unrecognizable as what it once had been, toward his unknowing mother, pulled the barrel of her shotgun into where his mouth had been and gargled “Please” up at her, it never even crossed her mind that what she was shooting was human anymore, let alone her son.
Not that she’d known who he was; it was Calvin she would mourn. Marty died alone and afraid, killed by a mother who never even knew her son, much as Doc Brown would die alone and afraid, thirty years later, at the hands of Libyan terrorists, unwarned. A shame, that such a tiny thing could cause matters to go so horribly wrong, but the moment Marty and the insect travelled through time together his fate was set, the mutation had already begun, and there was nothing he could do, no step he could take, to save himself.
And, by the end, he knew it.
By the end, he had truly become: The McFly.