Thinking back about that Alexander Wales post where he described a plot for a serial killer story but wasn’t sure how to end it. Between the different people who responded, I think we proposed every possible ending.
One proposed the simplest answer. The MMC gives in to his urges and kills the FMC.
I proposed the opposite answer. The MMC is in a perfect position to kill the FMC, but doesn’t, and realizes he never will. His morals will always win out over his desires, even if that means he can’t be happy.
A third person proposed the edge of the coin. The MMC realizes how close he came to murdering the FMC, they commiserate over how fucked up they both are, and things continue the same way indefinitely. The coin may flip one way or the other someday, or they may just continue like this for the rest of their natural lives.
Most of what I know about how to write a story is to create these three possibilities and make the MC choose between them.