The guy assaulted him first. That makes this self-defense. Murder not found.
Justification does not make a criminal use of force lawful; if the use of force is justified, it cannot be criminal at all. ... The defense of justification affirmatively permits the use of force under certain circumstances. ... The defense does not operate to 'excuse' a criminal act, nor does it negate a particular element of a crime. Rather, by recognizing the use of force to be privileged under certain circumstances, it renders such conduct entirely lawful.
The defense of justification would fail, for example, if a defendant deliberately killed a petty thief who did not commit robbery and who
did not appear to be a physical threat. However, the owner or lawful possessor of property has a privilege to
use any degree of non-deadly force necessary to protect his possession or recover his property, regardless of no physical threat to his person.
However, when an assailant ceases to be a threat (e.g. by being tackled and restrained,
surrendering, or fleeing), the defense of justification will fail if the defending party presses on to attack or to punish beyond imposing physical restraint.
A somewhat less obvious application of this rule is that admitting the use of deadly force in an attempt to disable rather than kill the assailant can be construed as evidence that the defendant wasn't yet in enough danger to justify lethal force in the first place.
In some countries and U.S. states, the concept of "pre-emptive" self defense is limited by a requirement that the
threat be imminent. Thus, lawful "pre-emptive" self defense is simply the act of landing the first-blow in a situation that has reached a point of no hope for de-escalation or escape.