The common AR of homicide offences has three elements. "The unlawful killing of a human in being". Unlawful is that is not lawful e. g. an execution like in the case of the British Hostage in Iraq Ken Bigley. Death is given its normal meaning and a human in is any person who is physically independent of their mother e. g. in R v Smith the soldier dies as a result of the stabbing. In this question clearly Dave is dead, his death is unlawful and he is a human being, therefore the AR is present. The MR of murder is the intention to kill or the intention to cause serious injury.
The simple direction for intention is the D wanting to kill. It is unclear whether Eddie 'wants' to kill. The courts have developed a test for indirect intent – were a D wants one thing but in order to achieve this something else will also occur. E. g. in R v Nedrick where the D wanted to scare a rival resulting in the death of a child. The test developed in R v Nedrick as amended in R v Woolin is; was death or really serious injury virtually certain consequence of D's act and did the D realise.
In Eddie's case it is clear that a heavy blow to the head with a spanner must at least result in serious injury and I believe the jury will decide that he recognised this and that they will infer from this intention. However the final decision is the injuries. Therefore, I conclude that the AR and the MR of murder are present and that Eddie will be liable for Dave's murder unless he can use a general or partial defence. Self-Defence is a general defence to murder. It has two elements.
Firstly was the force necessary, there must be an immediate threat of harm e. g.in Attorney General Reference No 2 where the D owned a shop in an area where there had been extensive rioting. He made up some petrol bombs, and kept them ready to defend this property if required. The court found that a defence was available to him as the threat was sufficiently imminent; even though not immediate. Also if there is a possibility of retreat it should generally be taken, however, the D or V's action must be measure as if you yourself were in there position. E. g. In R v Palmer the V may have been able to retreat but due to fright or adrenalin they froze.
There must also not be self-induced, such as in the case of Malnik v DPP where the D went to visit a man who was believed to have stolen some valuable cars belonging to an acquaintance of the D. The suspected thief was known to be violent, so the D took a 'rice-flail' (like a sickle) with him. He was arrested while approaching the man's house and the court REJECTED the argument that carrying the weapon was justified because he was in imminent danger of attack pointing out that he had himself created the dangerous situation by choosing to go to the man's house.
Evan if the D is mistake about where the force was necessary, the courts will view the facts as the D believed to be provide the mistake is genuine and reasonable e. g. in R v William (Gladstone) that hero #1 was attacking the youth. In Eddie's case the threat of harm from Dave is immediate, and there was no possibility of retreat due to the way in which Dave has his hands around Eddie's throat. Therefore Eddie's has the first element of self-defence. The second element required for self-defence is the question of how much force should and can be use.
This is simply that only reasonable force is allowed to be used. What is reasonable in those circumstances? What is reasonable varies from situation to situation and is decided by the jury. E. g. In the case of R v Martin it was held the D use excessive force by shooting the V in the back as he was fleeing the seen, because the youth pose no immediate threat. In Eddie's case I believe the jury will investigate whether an 'ordinary' male of Eddie's age coming under and attack from Dave would have reacted in the same way as Eddie's in the his instance of two body blows with a spanner.
However, following his Eddie's induced the fatal blow to Dave 'whilst he was staggering backwards from the blows' which I believe the jury would perceive as being excessive force and therefore not the reasonable force that an 'ordinary' for deliver, but the decision is for the jury. If the jury does reach this conclusion Eddie would be liable for the murder of Dave give the judge no alterative but to sentence Eddie to the mandatory life imprisonment. If they disagree with this Eddie will no be liable for murder and would be found not guilty and acquitted of all charges.