The beer scene. Lester and Carolyn make out in the living room, but before it progresses to sex, he almost spills his beer and she stops everything to chide him for it, and he utterly blows up at her for being so materialistic. Needless to say, the mood is killed. The fact that neither of them could just set the beer aside and go back to making love neatly spelled out everything wrong with their marriage.
At the end of the film, Carolyn was coming home with the absolute intention of murdering Lester. However, upon arriving home, she found him already dead in the same manner she had intended to kill him. In that moment, she realized the difference between the satisfying, cathartic revenge and subsequent freedom she had been imagining and the reality: that a man she had once loved, and still cared for, had had his life taken away and that she would never have the chance to reconcile or repair anything. This is why she ends up wailing on the floor, clutching her husband's shirts. Perhaps if it had come down to it, Carolyn would have realized this on her own and stopped herself from killing him...but before she could have that revelation, someone took it from her. In short, Carolyn's moment of discovering her husband's body was her more brutal, violent version of the revelation Lester had in the kitchen when he realized that Angela was a vulnerable young girl and that he was on the verge of doing something terrible that he could never take back. Lester needed only to imagine how terrible it would be to take advantage of Angela in order to stop himself. Carolyn never had the chance to stop it but must live with the consequences anyway.
Assuming, of course, Frank gets caught, but if he ever did, what would happen to Barbara? The woman is a shell, incapable of any thought for herself, if Frank ever left, since Ricky was booted...what would happen to her?
There is bound to be a large police investigation, and it won't take long for them to find the video of Jane and Ricky discussing killing Lester - such an incriminating tape could so easily be used against them as evidence and in the original script, it was probably the tape that sealed their conviction - the camera was turned off before they confirmed to each other that they were joking. They will probably also find Ricky's stash of cannabis, which will work against him too. Their only hope is if Frank owns up or they find his bloodied shirt - both unlikely (the latter is easily disposed of).
I doubt she was like that before the marriage. It takes a lot of effort, but even women who have been brutally abused (as is implied to have happened with Barbara) can make a recovery from the harm done to them. She'll undoubtedly have trauma issues for the rest of her life, but she's not irrecoverable.
It is highly likely that, if Frank were to get caught and hauled off, his wife would "come out of her shell", with the lack of the constantly dominant and likely oppressing presence of his. It might take time, but she'd turn out okay.
Almost all of the rest of the cast have a reason to have committed Lester's murder and all of their dirty laundry is going to come out in the ensuing criminal investigation.
On first sight years ago it seemed that the reason Frank did it was either because he was rejected or that he still believed Lester was gay and had hurt Ricky, just liked younger boys. Just realized it was because he couldn't let Lester live after the kiss, because it meant that someone knew Frank was closeted.