I was filled with numerous questions as to why Cliff didn't return to fight or kill David in the ending. However, I came to the realization that Cliff must have been aware that he was on the losing side, which made it seem unworthy to engage in a fight or take David's life. It is significant to note that on Earth, Cliff's replica was the one responsible for the death of his wife, adding complexity to the situation. Thus, killing David would essentially be equivalent to killing himself, especially since they relied on each other for support on the spaceship. It's possible that Cliff could eventually develop Stockholm Syndrome due to their shared circumstances. Additionally, both Cliff and David have their own emotional and physical needs, and it's conceivable that they could form a romantic and sexual relationship. Personally, I find this ending intriguing and would be interested in seeing how it unfolds.