The knife she had when she turned herself in didn't have the mark so it's clearly part of a bigger plan. I dunno what the purpose behind stabbing the body was if she was gonna use a different knife and the original was already covered in blood. It makes sense she cut up and disposed of the body otherwise it would clearly be suicide, they've been able to tell since the 1800s if stab wounds were inflicted post mortem. Maybe it was so she'd have bloody clothes?