What is the necessary (a) actus reus and (b) mens rea to establish guilt of murder?

The answer we are looking for here is : (a) the unlawful killing of another human and (b) malice afterthought. Criminal law always requires two elements in order to find an individual (D) guilty of a crime : he must have commited a "guilty act", which in latin translates to actus reus, and must have had a "guilty mind" which translates to mens rea. So here, for D to be guilty of murder, he must have had both the mens rea and actus reus of the crime.Of course, it does appear quite obvious that the guilty act in murder is the act of killing someone. This is usually the easiest element to identify.The mens rea, the guilty mind requirement, for murder, is that it must be proved that D intented to kill or cause grievous bodily harm to his victim (V). But what if D was acting in self-defence? What if V had been coming at him with a knife, and in that moment it was kill or be killed? Would this in any way affect whether D had the necessary mens rea or actus reus to be guilty of murder?The answer is yes. Self-defence is indeed a defence at common law, in fact it is called a "justification". Pleading self-defence means that you are trying to disprove the actus reus. By pleading self-defence to murder, D is saying : yes, I did kill V, but he was about to kill me, I had no choice. I must to be allowed to save my own life. And common law agrees. The law of England and Wales recognises that if you are in danger you must be able to defend yourself as needed, thus if you do defend yourself, whatever act you do is not in fact "guilty". You lack the actus reus and cannot be found guilty of murder.