Rick and Morty embraces a somewhat pessimistic morality, rejecting ideas like destiny and fate for a colder, more chaotic and random universe. But there still seem to be some constants, largely regardless of what universe they exist in. A new theory about the show suggests that perhaps that's because there is an eventual fate for Rick and Morty -- a particularly dour one that could even explain the machinations of malevolent figures like Evil Morty.
This Rick and Morty fan theory suggests that the titular pair are trapped in a time loop, where Morty will eventually grow up to become Rick.
The theory, which was proposed by user u/lukeMGH on Reddit, suggests the real reason Rick is so bitter and angry is that he's aware he's trapped in a time loop. Rick is actually a version of Morty who grew up and developed into the same cynical super-genius that took him across the multiverse in the first place. Eventually discovering this truth after either the disappearance or demise of Rick, Morty assumed the name Rick Sanchez and traveled the galaxy before getting his own version of Rick's life -- eventually meeting a woman, having a daughter named Beth and setting the cycle to begin anew. This is why Rick is so miserable with life -- he knows eventually he'll either die or disappear, leaving his grandson to suffer the same sadness he always has.
If this is true, it might explain the apparent near-multiversal constant of Ricks and Mortys existing together. Having this be the fate of all versions of Morty makes it even more tragic, as it seems to be facilitated by the Citadel of Ricks. At the multiversal base, the Mortys that actually survive long enough to grow up end up darkened by their exposure to their adventuring grandfather. Even the Mortys who lose their Ricks aren't safe from his impact. They either end up abandoned in their home reality with the knowledge that their grandfather is dead or are taken back to the Citadel to be trained to assist a different version of Rick. The ones that don't fit in live on the streets of Citadel, typically dying pointless and forgotten deaths.
Notably, this theory would mean that Morty's epiphany from the first season was actually false. After dooming their home reality during "Rick Potion No. 9", Rick and Morty traveled to a dimension very similar to their own where this world's Rick and Morty suddenly died. After burying his own body and taking his place in the world, Morty comes to the conclusion that "nobody exists on purpose, nobody belongs anywhere, everybody's gonna die." But as he famously tells Summer during "Rixty Minutes," Morty still finds some happiness in his loved ones, even if he has no destiny. It was established as an important moment for the character, especially given how dark his adventures with Rick have gotten.
But if Morty is fated to grow into Rick, then his belief that he doesn't matter in the long-run doesn't hold up. Morty does have a purpose -- to become Rick and keep the circular timeline running. That's a depressing destiny for the character, especially since he's become more and more disillusioned with Rick as the series has progressed. This could even be what led to the birth of "Evil Morty" as we've seen him, a version of Morty who was actually smart enough (and brutal enough) to kill numerous Ricks and steal and torture their Mortys.
Finding out that the universe has a cruel design for every version of Morty could have even driven Evil Morty to try to do something about it. This could be his motivation for taking over the Citadel of Ricks during "The Ricklantis Mixup," setting up a new system where not only are Mortys free to accomplish other goals or live their lives but also to break the cycle of becoming Ricks.
This would give the villainous figure a genuinely compelling motivation that might even have some sway with the core-Morty, who has a bit of long-standing anger toward his grandfather. Breaking the Morty-becoming-Rick cycle could be the ultimate way to make sure that no one truly has a destiny for real, not even Rick. It's an interesting theory that would upend the entire mythology of the show, and put Morty into an impossible situation.