My opinion on the sock, and its just my opinion....
I think she used the sock to stab the boys, then ran it down the alley to get rid of it ( I dont think she thought they would actually look that far down the alley for any evidence, if she was planting it I think she would have done it closer to the house), then decided to cut herself ( I dont think this was planned in the beginning ) and realized Damon was still alive and had to stab him the second time.
Like I said, its just my opinion.
I think she used the sock to stab the boys, then ran it down the alley to get rid of it ( I dont think she thought they would actually look that far down the alley for any evidence, if she was planting it I think she would have done it closer to the house), then decided to cut herself ( I dont think this was planned in the beginning ) and realized Damon was still alive and had to stab him the second time.
Like I said, its just my opinion.