Fan Theory: Rick accidentally killed Morty Prime when testing the “save point” device.

In the episode where Morty asks Rick to invent a machine that can “save your spot in time” and later reload it, Rick initially lies and says he couldn’t build it.
Morty replies, “No, it’s okay. Maybe we can start by inventing a little honesty.”
After that, Rick secretly activates the device by creating a save point and jokes, “You’re right… or maybe I’ll just kill you!” He then shoots Morty. Immediately afterward, he presses the red button, returns to the save point, and shouts, “BECAUSE I FUCKING DID IT!”
At first, this looks like Rick simply rewound time. However, at the end of the episode, Rick reveals that’s not how the device actually works. He explains that it doesn’t rewind time at all (which would attract the Time Cops). Instead, every save point searches for hundreds of thousands of perfectly identical universes at that exact moment. When you load a save, the machine finds one where the user has just died of natural causes and transfers your consciousness there, making it appear as if you rewound time.
If that’s true, then the very first time Rick tested the device, he wasn’t transferred anywhere—he simply shot the Morty standing next to him. That Morty never came back because there was no previous save to load.
If this interpretation is correct, Rick permanently killed Morty Prime during the very first test of the device, and every Morty we see afterward is technically from a different, otherwise identical universe.

(Note PLEASE READ THIS BEFORE TRYING TO DISPROVE THEORY)

Some may argue that one of the Rick’s sent Morty to his original universe, however we know that’s not the original Morty and just a copy of Morty because Rick still presses save and loaded, killing original Morty which loaded another version of Morty (we will call this Morty “Morty Prime V2” to not get confused)
Then this Morty was the Morty that Rick gave the device and went on a bunch of adventures with, and Morty Prime V2 was the one who went to the unspecified ricks universe, and was sent to universe C-137.