Throughout Rick and Morty's run, one of the main themes has been that of revenge. As the seasons progressed, this actually became the main reason for Rick's existence. Amid all his kooky journeys with Morty across various realities, fans would learn that this Rick (from the C-137 dimension) was really hunting Rick Prime all along for murdering his family.
Source：https://www.cbr.com/rick-and-morty-unmortricken-post-credits-scene/
