I think Stella kept the note as a reminder that love, not anger and hate, is the path to redemption. That resonates with the discussion she had with young Katie too. I thought that scene was perhaps the best of this series.
Over all, I felt that Series 3 was quite weak. A bit like the second series of Broadchurch, it had far too many implausible breaches in procedure for dramatic effect- the lax supervision, the hospital scenes...is there only one doctor in Belfast? And why all the oddly intrusive personal questions to Stella? It certainly seemed like the scriptwriter had swallowed a medical dictionary too, and felt he couldn't bear to waste even the smallest part of his research- we really didn't need that level of detail on a TV show: I could practically perform a splenectomy myself now.
I can't decide whether I liked the many feminist touches- women being talked of as stronger than men, the encouragement Stella gives her female colleague, buying her own flowers etc etc- or whether they were just a bit of a sop thrown to the viewer after so many episodes of hyper-sexualised female corpses.
I did like the sensitive pep talk Stella gave to Katie, and I gave a small cheer for the female solicitor in the taxi telling her odious colleague that she couldn't stand to be in the same room as Spector any more. Good for her!
It was a compelling drama over all, mainly due to the performances, but definitely very flawed, and probably stretched out too long for the material. I would have liked this series to draw some conclusions about the police corruption hinted at in the beginning, but that entire subplot seemed to go nowhere.
The final message appears to be that if life is a fairytale, it is Grimm indeed. But that the Blouse, in the end, will triumph over the Balaclava.