I’ve got a fan theory.
We know in an alternate universe, Beth never ended up with Jerry. She was successful and miserable until the alternate Jerry showed up. As the separation in this season continues, Beth will see herself becoming more and more cynical like Rick. Then we’ll see her memory of when her mom left Rick. It’s obvious at this point that we won’t get an accurate origin story from Rick. His cynicism ended the relationship. Presenting this, Beth will some how seek out Jerry because being with him keeps her from ending up like Rick. And that’s where Evil Morty comes in….
He’s from an alternate universe where Beth and Jerry split for good. And the cycle of cynicism continues with a darker Morty.
Besides, she’s no idiot. She’s going to figure out that Rick manipulated her divorce from Jerry.
Something was bugging me. What could possibly be Evil Morty’ s motivations for, well, being evil? They came to me when I was mowing the lawn this morning. We can assume that Evil Morty’s circumstances are similar to Morty C-137. But without Jerry’s idiotic optimism. If you watch previous episodes, you’ll not that every time Morty tries to do the right thing, or take initiative, or outsmart Rick, he gets slapped in the face, metaphorically. He’s a character who wants his actions to matter. Now, in a universe where Rick’s cynicism passes to Morty, and armed with the knowledge that nothing he does matters, he craves to be unique and to finally matter. How does he achieve this? By wiping out every universe that bears resemblance to his own, and every other Morty in the process. Only then can he truly matter and be unique. He probably recognizes that Rick Sanchez C-137 is the only one who can stop him because he’s the “Rickiest Rick of them all.”