Here's my theory in a nutshell:
I think she and Darin had a huge blowout when he returned from taking Dana home. This is why she's downstairs and not in bed with Darin. Darlie was jealous, thinking something was going on because it took Darin so long. This, on top of mounting financial concerns, a slowing business, weight issues and depression took a toll. In looking at the evidence, her wedding dress was out if I remember correctly, picture albums were out as though she had been going through them, I think there was even a blood smudge on one, insurance policies were out .... as though she was reminiscing. If you look at her history, she had a habit of the dramatic when things weren't going her way. Like coming back to Darin's graduation party claiming someone tried to rape her so he'd leave the party with her. Anyway, I think this was the final plan. She wanted to do something extreme. She takes the sock out of the laundry room and puts it on her hand. She goes to the kitchen and takes out the bread knife first. Goes outside and cuts the screen. Takes off her underwear and throws them somewhere (not sure what happened to them, possibly storm sewer). Puts the bread knife back, takes out a different knife and commits the crime against the boys. Runs out the back, down the alley and throws the knife and possibly the underwear down the storm sewer, dropping the sock. Runs back to the crime scene where she finds Damon crawling towards the door. She goes back to the kitchen, grabs the big knife and finishes the job. Now she decides to cut herself to make it look like they were attacked by someone else. After a couple hesitations, she finally cuts herself good. There's blood everywhere so she hurriedly cleans the sink, opening the cabinet below possibly to retrieve something to clean with, which is why there is blood dripping down the inside of the cabinet doors under the sink. By this time she's bleeding pretty good. She has to make it appear that there was a struggle so she tips over the coffee table (probably her unidentified, smeared bloody print), wipes Damon's handprint off the couch, drops the flower arrangement on the floor, goes to the kitchen and tips the vacuum and finally drops the wine glass. Darin claimed he heard the glass break, but who knows. The rest is history.