Punches are generally non deadly force. In this case The roundhouse thrown certainly was non deadly force. You cannot respond to non deadly force with deadly force. As a retired attorney who practiced in both Mass and New Hampshire I can tell you that the arguments regarding the defendant thinking he may have received life-threatening damage from the knockout which would then permit him to use lethal force is not going to fly in front of a jury. This is because of two reasons, the first is a reasonable person would not think that and that is the standard. The second is when you're not in your home, and you are confronted by deadly force you have a duty to retreat if you can. And these hold true in both civil and criminal law. Last, somebody said something about John's size. Well it's not definitive, again, go to the reasonable person. What a reasonable person of John's size be more or less fearful of someone much smaller? (Yes, someone's always going to bring up what if John thought the smaller guy might be a karate expert or something. Again, go to the reasonable person standard, would someone thinking the smaller guy is a martial arts expert be a reasonable assumption? No. That would be a big leap of faith). The ONLY a thing that might possibly make him guilty of a charge lesser than second degree murder is having received a blow to the head minutes earlier. That would be the battle of the expert witnesses. Second degree murder is not what they call a specific intent crime it is a crime of malice, therefore voluntary intoxication, which is a defense against specific intent crimes, is not a defense here in any way to lessen the charges.