I've been reading through the threads on this case again after seeing it featured on a Forensic Files episode this week. Just my opinion, but there are easier ways to get rid of a sock than taking it down the street, so I feel Darlie was not attempting to dispose of the sock. If she simply wanted to get rid of it as evidence, it would most likely have flushed easily down the commode. (You wouldn't believe the odd things that kids/family members have flushed down the commode at our house, including a whole hand towel once, haha.) I think it may have been planted to lead LE to look for suspects outside the Routier's own house, as if the perpetrator dropped the sock while he/she fled.
However, in regard to Darlie's skin cells being found in the sock, you would probably also find my skin cells inside a good deal of the clothing in my household since I'm the one who does the laundry. Socks often end up inside out and need to be turned, so it could be that when Darlie retrieved the sock from the dryer, she inserted her hand and pulled the sock right side out, leaving some loose skin cells behind. Having said that, my opinion falls more on the "Darlie Did It" side than that a stranger did it. This crime makes no sense. No robbery, no sexual assault, no true motive for it.
Putting the sock over the knife handle could prevent fingerprints but also might provide a better grip on the knife. If the killer was Darlie, she couldn't know whether the knife handle might become slippery during the crime, either from a sweaty palm or from blood.
I would like to think a mom couldn't do such a thing as this but we see bizarre heinous crimes in the news every day.