I disagree, but in the end it's not for you or I to decide, it's for the jury to decide, they're the ones who have to consider the evidence and work out whether the defendant's actions were reasonable or not.
Simple head injuries can result in TBI or even death. Add on the leverage and weight from a skateboard and that very quickly becomes a deadly weapon.
You seriously underestimate how easy it is to kill with blows to the head. Please, do some research. You clearly have no clue how many 'murderers' are in jail because they got in a fist fight that went horribly wrong with just one strike.