Unlawful killing of a human being
‘Homicide’ could briefly be explained as the unlawful killing of human a being. There are three main points to ascertain in order to establish ‘homicide.’ The first step is to determine whether the defendant has committed the actus reus, which is the physical element of a crime. This could be either by an act or an omission. (Gibbins v Proctor   ). For an accused person to actually be liable, he/she must have acted in free will or voluntarily. In this case there is no doubt that when Walter hit Angela he was in full control of his actions.
However, this itself is not enough. The defendant must be proved to have caused the death of the victim (it still does not have to be the only cause of death). The two points to be considered are, if the defendants in fact caused the death of the victim and if so is he also the legal cause for Angela’s death? It also has to be proved if any intervening act broke the chain of causation. Factual causation is measured by the ‘but for test’ as in the case of R v White  . But for Walter striking Angela, would she have died? The most obvious answer for this would be ‘no’.
To satisfy legal causation, defendant cannot be held liable if his actions are too remote from the death or where they are only a minimal cause of death. As for the decisions made in cases such as R v Smith   and R v Jordan  , the defendants act must be the substantial and the operative cause of victims’ death. But the later decided case of R v Cheshire   said that the defendants act need not be the substantial cause of death, but must be more than a minimal contribution (R v Cato   ). However, in this case it is quite evident that Walters actions are a cause (which in effect is more than minimal) of Angela’s death.
One could argue that the doctors’ actions were the actual reason why Angela ended up losing her life and that this may break the chain of causation. But I firmly believe otherwise. As decided in the cases of R v Jordan  and R v Cheshire  medical treatment is very unlikely to break the chain of causation as long as it is not independent of the defendants act. As per the text, there is no evidence to ascertain that the doctor could have known of the victims’ allergies and therefore I believe he could not have foreseen the result.
In collection of all of the above, I am very much convinced that the defendant is in fact the cause of the victim’s death.
The next step in the venture of establishing a ‘homicide’ is to see if the defendant actually did fulfil the mental element of the crime known as ‘mens rea’. Mens rea for murder is; intention (to kill or cause ‘Grievous Bodily Harm’ (GBH)). If however, Walter is found to have fulfilled mens rea, then he most likely would be found guilty of murder. (Murder is defined as the unlawful killing of a human being under the Queen’s peace with malice aforethought) Therefore I believe that this essentially is the most important factor to be considered.
When he was questioned by Police, Walter says that “he was felt compelled to hit her because voices in his head were telling him that she should be severely punished for her behaviour  ". Therefore even though he may not have decided by himself to strike her, after hearing the ‘voices’ he did strike her with the intention of causing her severe harm.
“Traditionally the mens rea of murder was described as malice aforethought but the term is unhelpful: ‘malice’ does not mean ill will and ‘aforethought’ tells us no more than that the mens rea must not come as an afterthought. It is simply a label used to refer to those mental states (whatever they are) that suffice for murder. The current law is that the definition of malice aforethought is satisfied by proof of an intention (direct or oblique) to kill or do really serious harm." 
As per the case of R v Moloney ,  foresight of consequences is evidence of intention. Intention could be either direct or oblique (indirect). Direct intent would be the aim/purpose to kill or cause GBH where as oblique intent is to act with the knowledge that death or GBH would be a virtual certainty of his actions. R v Vickers   confirms that intention to cause GBH is sufficient mens rea for murder. In the more recent case of Hancock and Shankland   it was said that ‘probability is important in deciding whether a consequence was intended. Also as in R v Nedrick   and (more recent) case of R v Woollin   , where the consequence was a virtual certainty and the defendant realised this, the jury is able to establish intention.
However in this instance of Walter, striking Angela, he did act with the intention to cause her GBH. He also should have therefore realised the consequences that his actions would bring about. This would mean that Walter has sufficient mens rea for murder.
Once a person is convicted for murder, the only defence available to that person would be under ‘Voluntary manslaughter’. This could be with regards to issues like provocation, diminished responsibility etc. In this case the only defence that would be available to Walter (or rather the best option under the circumstances) is ‘diminished responsibility’.
Diminished responsibility is a statutory partial defence and it applies only to murder. Before the 1957 act, the condition of mentally ill defendants were not taken into account by courts unless in circumstances where the mental illness satisfied legal ‘insanity’. Diminished responsibility was first introduced after the Royal Commission on Capital Punishment as a compromise solution to the above problem. This defence however reduces liability from murder to manslaughter and gives the judge the discretion to sentence.
Diminished responsibility is defined in s 2(1) of the Homicide Act 1957. It provides that, a person shall not be convicted of murder,
“If he was suffering from such abnormality of mind (whether arising from a condition of arrested or retarded development of mind or any inherent causes or induced by disease or injury) as substantially impaired his mental responsibility for his acts or omissions in doing or being a part to the killing" 
If Walter plead diminish responsibility, the burden of proof would be on him to prove the matter on a balance of probabilities(R v Dunbar   ). The three elements of diminished responsibility are(as above), abnormality of mind, resulting from a condition (as explained in s2(1) above), that substantially impaired his responsibility for the killing. In R v Byrne   the ‘abnormality of mind’ was described as a state of mind so different that of ordinary human beings that the reasonable man would term it abnormal.  It also does cover irresistible impulse. Byrne also conclude that it was being on the borderlines of insanity. But as corrected later, comparisons with insanity should be avoided. (R v Seers ).  According to Seers, the word ‘substantial’ means more than trivial but not total or absolute impairment.
According to research conducted by Professor Mackay, the second most frequent diagnosis used in connection with diminished responsibility plea was schizophrenia. The possible causes of abnormality of mind are restricted by s2. It is for the defendant to show that this abnormality did in fact fall within those specific categories. This should also be supported by medical evidence.
The abnormality of mind must have substantially impaired the defendant’s mental responsibility. As in the case of Lloyd ,  this does not have to be a total amount but can also be trivial or minimal. It however, is a question of degree. It is a matter for the jury to decide whether the defendant’s responsibility was impaired  .
Diminished responsibility must not be left for the jury unless there is medical evidence to support the three elements (R v Dix )  . Where the medical evidence submitted by the defendant is not disputed, a plea of guilt for diminished responsibility may be accepted by the judge (R v Cox   ).
Over the years, diminished responsibility has been a very popular defence for murder. Between 1997 and 2001 a total of 157 defendants raised diminished responsibility as a defence. Out of these, 8 contested and 118 uncontested pleas were successful making it a total of 126 out of 157. Approximately about half of the successful defendants were hospitalised under the mental Health Act of 1983. 
Courts should however take into account all the evidence and if the medical evidence is not contested and there are no other doubtful facts, they are bound to accept it. (Byrne )
“It was never intended that pleas should be accepted on flimsy grounds ............. cases are tried by the Courts and not by psychiatrists. It seems to us that pleas to manslaughter on the grounds of diminished responsibility should only be accepted when there is clear evidence of mental imbalance". Lawton LJ in R v VINAGRE  
As the question text suggests, the defendant, Walter suffers from schizophrenia. Provided that this is a clinical diagnosis of his medical condition and also providing that there is no doubtful facts related to this diagnosis, nor any other fact related to his medical condition that could be raised in courts, Walter should have a valid defence against his actions. With regards to all of the above I believe in my opinion that Walter should be able to successfully plea for the defence of diminished responsibility and thereby reduce his liability from murder to involuntary manslaughter. This should give the judge the discretion as to sentence.
There is large criticism over the fact that the defence of Diminished responsibility is not precise and that the outcome largely relies on jury sympathy and also the willingness of psychiatrists to prepare their testimony in line with the legal requirements. Even though these criticisms were acknowledged, the Law Commission in its report in 2004 “partial defences to murder" stated that as long as the mandatory sentence remained the defence should be retained.