The actus reus of Murder therefore requires that the defendant should have caused the death of the victim through an act of his/her own. Death should be caused within the Queen's peace and three years of the act of the defendant. In this question the defendant Zak has killed Julie. Murder is a consequence crime and therefore accordingly requires causation to be proved in order for the actus reus of murder to be proved. It is in essence proving that it was the act of the defendant that caused the death of the victim. Causation is a 2-stage test and requires firstly causation in fact. The test for causation in fact is the 'But for test' (but for the act of the defendant, would the victim still have suffered the consequences and if not then there is causation in fact). The element that has to be proved is that it was the act of the defendant that put the victim in a certain setting; in which he would not have been but for the act of the defendant. In this instance it is not difficult to prove causation in fact, as had it not been the act of Zak, Julie would not have died. Once causation in fact is proved the second test of causation has to be satisfied.
The second stage of the causation test is causation in law. ...
In this question it seems that in relation to death of Julie there is causation in law as the act of Zak was the operative and substantial cause of Julie's death. Neither had the act of Zak exhausted its effect and further it was also the significant and sole cause of Julie's death. It seems therefore that the actus reus of the offence of murder is proved as Julie dies in the fire and the place of his death is within the Queen's peace.
Mens rea for the Murder or the lack thereof
This takes us to the next issue in the question, whether Zak had the requisite mens rea for murder. The mens rea for murder is intention to kill (express malice) or cause grievous bodily harm (implied malice). Traditionally the mens rea for murder is called "malice aforethought". In Smith & Hogan Criminal Law 9th Edition malice aforethought has been defined as:
'a mere arbitrary symbol for the 'malice' may have in it nothing really malicious; and need never be really ' aforethought'.
Therefore the requirement today is that the defendant should have intended either death or grievous bodily harm as a result of his/her act. Malice aforethought is generally taken to mean that the defendant should have intention to bring about either of those two consequences. Intention can be defined as the either the purpose of the defendant's act or even if it not the purpose of the defendant's act, intention can be inferred from certain subjective foresight on part of the defendant. In other words if the defendant realizes that the consequences are virtually certain as a result of his act then the courts can hold that the defendant intended the consequences as a result of his act.
The problem here is that Mens Rea for this murder seems doubtful as it has been stated in the question.