it’s not really that simple though. there are different forms of manslaughter, some result from partial defences to murder.
and the same sort of thing could be said about lots of offences. should people be charged with something as serious as causing death by dangerous driving every time they do something silly when driving? we use the ones who do end up killing through their culpable actions as a deterrent and you could argue there is good reason not to take account of a criminal’s moral bad luck. if you are recklessly or deliberately violent then you are taking on that onus already and if you are unlucky then it is just too bad. level of intent doesn’t have to perfectly correspond to the extent that someone is held responsible.