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.
Lester Burnham =Humbert Learns. Literally floored me when I learned it. In 2012.