If you come up behind someone and give them a beating, your intention is certainly to seriously hurt them and anyone with all their mental faculties knows that a common side effect of being seriously hurt is dying. He had to know when he landed the first punch that there was a chance the guy could die. Despite knowing that he still went ahead and gave him a beating, that's the difference between coming up behind someone and just getting drawn into a fight
IMO. You have time to consider your actions and the consequences of them, if despite all that you still go ahead with it, then you should take the punishment. The defendants true intentions will never come out in court anyway, as soon as he see's a police officier it will all be a huge mistake etc etc.
Should the courts make exceptions because someone was easier to kill than the defendant thought they would be?