So, I have this headcanon that on Earth-X, Sara and Alex were together secretly and when Earth-X Quentin found out that Sara wasn't straight he had her killed and so to get revenge Alex attacked Kara and Kara killed her.
Let me know your thoughts.
P.S. I don't ship Alex/Sara on Earth 1 or 38. The reason I picked Alex is because Ava is from the future and Nyssa is on the other side of the world.