A classic noir plot involves one partner murdering their spouse for the purpose of marrying another, perhaps someone they love more. (Interestingly, the plot is done in both gender directions without either seeming like an inversion of a trope.)
Assuming this happens in real life, what is going on in society that sets up the incentives for murder? Obviously, living is so much better than dying that it is surprising that some sort of Pareto-improvement deal could not be worked out that avoids killing. It might be that the surrounding society prohibits or otherwise punishes divorce, though that still begs the question why, especially since such a practice incentivizes murder.
One possible such incentive, good for noir, is that the jilted first partner will kill the new second partner to eliminate the new competition, so the first partner must be killed to save the second: game theory.