There are several different types of manslaughter. There's a lot on the interweb about it if you want to get into details, but as a basic there is:
Manslaughter - diminished responsibility - basically your mental state (due to an "abnormality of the mind") meant that you were not full responsible for what you did, although you don't reach the threshold of full-on "legal insanity". You probably don't want to try for legal insanity as an excuse for murder because if the judge agrees and thinks you are insane then you can be locked up indefinitely which is more than you would usually get as a sentence for just straight murder, so this is the more common option if there is any suggestion of something not being right with a defendant's mental state.
Manslaughter - provocation - this isn't "he was trying to stab me so I stabbed him" as that would be self defence; this is the actions or words of the person pushed you to the point that you had a sudden and temporary loss of control. The jury have to consider that a reasonable person would have also been provoked. Examples used to be things like the other person confessing adultery to you, but more usually it would be something like domestic violence pushing someone to the edge.
Manslaughter - suicide pact - not relevant.
Involuntary manslaugther - due to "gross negligence" (normally something like corporate manslaughter such as a train crash, an accident at work) or an "unlawful act" where you intended to commit an unlawful act but death could not have reasonably been forseen - such as if you burgle the house of a frail OAP and they are shocked into a heart attack then that would be a forseeable event, but if you shock a healthy young adult it might not be.
Until the prosecution finish and the defence start we won't get "his side of the story", but I would expect diminished responsibility might be quite likely - although he might claim he only intended to break into her flat and panicked when he found her there or something weird like that, but it doesn't sound too likely at present based on the description of the injuries she suffered.
I observed a murder trial in Cardiff years ago where the defendant was a teenage girl who had killed her boyfriend after he slept with a mutual friend - he told her what he'd done and she grabbed a knife from the kitchen side and stabbed him. She had suffered depression and had clearly had a pretty messed up life before this, and the jury found her not guilty of murder but guilty of DR manslaughter. I thought at the time it might have been the jury just deciding to show her some sympathy by picking the verdict that would have a significantly shorter sentence, since murder is automatic life (thought rarely more than 10 or so years) and manslaughter is discretionary - I think she was given a sentence of about six years so she would be out by now.