Part of what makes Back to the Future such a timeless film is the enduring friendship between Doc Brown and Marty McFly. Without wasting any time, it's clear that these two are the best of friends solely because Doc trusts Marty enough to show off his experimental time-traveling DeLorean. Of course, considering the scientific breakthrough that a machine of that magnitude carries, there's nothing more important than trusting who is told. That being said, even though the two share a strong bond, there are still theories that Doc wouldn't hesitate to make a hard choice to save reality.
Back to the Future follows the adventures of Marty McFly after he is transported via time machine to 1955. Once there, he is caught in a predicament where he has to make sure his parents fall in love while getting a younger Doc Brown to help him. Once Marty returns to his time, he sees his past self go back to 1955, but one theory believes that the Marty going to the past was actually going to his death.
One of Back to the Future's biggest rules is that past or future versions of a person should never interact with their present selves. In doing so, there's a possibility this could lead to a paradox, destroying reality entirely. As a result, all of the films take creative measures to avoid seeing one another, aside from when Jennifer meets her older self in the future. One measure that's never confirmed is that Doc Brown sends Marty to certain doom, knowing that his future self would return at the exact same time, breaking a time loop and further avoiding a paradox.
The brutality of the decision would make Doc Brown seem more like his later inspiration Rick Sanchez from Rick and Morty. In that series, Rick has no problem making the hardest choices to save the whole of reality, but doing so makes him bitter and even depressed. Although it would seem this Doc Brown would eventually end up the same way, he is never shown committing such unspeakable acts again. In fact, his behavior begs the question, did he really send his Marty to a doomed reality?
According to an interview with Doc Brown actor Christopher Lloyd, something like that would never happen. In fact, he states, "...Doc would never send Marty off to his death, in any kind of scenario. Doc couldn't live with that." Since the first film, the franchise has shown that Doc would sooner sacrifice himself over everyone he loves, even staying in 1885 to ensure Marty would get home. Because of his kind nature, the scientist would never commit such a cold-blooded maneuver.
That being said, there have been alternate futures and multiple variants of Doc and Marty littered throughout the series. As a result, while the main Doc Brown would never hurt a fly, there's a chance that there's a Doc who would. In fact, it could even be one seen in the other Back to the Future films, like the one committed in the dark future where Biff Tannen is in charge. Nevertheless, while the theory is unlikely true, it does get the creative mind thinking.