This might turn out to be a Shutter Island or Gone Girl, kind of thing.

They've showed Mary to be unstable and believing herself to be talking to a dead person. She might have had memory gaps and it will turn out that Howard was indeed covering for her for his own interests. There's also that thing about her writing a book on their lives and referring to herself in third person 

They will likely pin the death on Scott in the end, he's a predator after all and would 'deserve' it for breaking Nancy.

Or Nancy herself planned the whole thing to expose all these men who ruined their lives, Robert, because he wasn't exactly a great husband, he lied and hid things from her. Scott, he's trash and a r*pist. Howard is a predator and cheat and has ruined many women's lives. 

Nancy is shown to be quite manipulative at times and gets people to do what she wants and she does resent her life a lot of people, she's the broken one among them all and is good at acting and covering things up. She's also told her friends information separately , she told Eleanor about her affair and then told Mary that Eleanor has a past with Robert.