I think if you knowingly engage in a violent action that's likely to result in death then it should be murder and not manslaughter
I think though by very definition that is what manslaughter is
The unjustifiable, inexcusable, and intentional killing of a human being without deliberation, premeditation, and malice. The unlawful killing of a human being without any deliberation, which may be involuntary, in the commission of a lawful act without due caution and circumspection.
I think part of the problem is that there are two distinct types of manslaughter (voluntary and involuntary). He tried to argue the latter he has been sentenced with the former.
legal-dictionary.thefreedictionary.com/Voluntary+manslaughter
Is a good read on it. It is a legal distinction as much as anything - though in practice comes out in slightly lower sentences. It does say though it shouldnt be seen as lesser to murder - he has been found by the judge GUILTY of killing her - just that mitigating circumstances have reduced his sentence.
Sadly there is no miscarriage of justice and no room for appeal. I think the Judge did very well within the confines of the law to give the sentence he did