I just wonder whether she was in a fit state to write anything. When and where did she write the note? What happened to the pen? Why were there no traces of blood in her room if she had killed everyone and written the note there?
Of course Sheila was capable of writing,though latterly her words would be jumbled as her mind had been.
Why Sheila has been made out to be this shy,timid,incapable ditsy person I don't know ?
Sheila was a prolific writer who preferred to communicate that way rather than speak her mind,or what she was thinking at the time. Drawing/painting her feelings was also something else that she did too as one of her drawings scared Colin when he first saw it because it depicted herself in the guise of Klimt's " scream " which is how she'd felt.
The notes could have been written in her bedroom-------when her hands were clean,but she'd appeared to have known what to write at the time to coincide with what went on.
There was no blood in her bedroom because it was the only room which wasn't involved in the killing of anyone.