I disagree with it, personally.
In my view, it was a premeditated act; she went equipped to kill and bludgeoned her victim in a gratuitously violent way.
As far as I understand it, the grounds for reducing her sentence to manslaughter was based on the notion that she was suffering from an undiagnosed mental illness at the time of the attack which gave her the diminished responsibility.
A change in the law meant that she could use this as new evidence.
She asked for a retrial, which was granted, but the CPS decided that the murder conviction should be dropped to manslaughter on grounds of diminished responsibility without her having to be retried. Not sure why the retrial was dropped (unless the CPS feared she'd be found guilty of murder again perhaps? )