Blood Simple (1985) on Netflix

Texas bar owner Julian Marty, who is generally regarded as not a nice person, hires shady private detective Loren Visser, who is able to obtain what Marty requests evidence - in this instance, photographic - that his wife, Abby, and one of his bartenders, Ray, are having an affair. As Ray and Abby realize that Marty has found out about them, it allows them to plan for their future away from Marty, while being up front with Marty about the situation. Marty, in turn, decides to hire Visser once again, this time to kill Abby and Ray, and dispose of their bodies so that they won't be found. The out-in-the-open affair and the contract hit lead to some actions based on self-interest and a standoff of sorts between the four players, which is compounded in complexity by some wrong assumptions of what has happened, with an innocent bystander, another of the Marty's bartenders, Meurice, potentially, and unwittingly, adding to the scenario.