@SoVery
You have to prove INTENT to murder. Not something that happened purely as a result of his actions.
The CPS says (my bold) :
Manslaughter can be committed in one of three ways:
1 Killing with the intent for murder but where a partial defence applies, namely loss of control, diminished responsibility or killing pursuant to a suicide pact.
2 Conduct that was grossly negligent given the risk of death, and did kill ("gross negligence manslaughter"); and
3 Conduct taking the form of an unlawful act involving a danger of some harm that resulted in death ("unlawful and dangerous act manslaughter").
So the court believed he intended to murder her but also accepted his plea of diminished responsibility. He had not sought any help for his depression and the only evidence he was suffering was from his daughter as far as I can tell who repeated the 'he wouldn't hurt a fly' line that he used.