Well in the US it depends on the state and the circumstances. Some states have castle law doctrines such that if someone attacks you in your home or car you are under no requirement to retreat. I generally agree with those laws.

On the other hand we don't allow settling of beefs. If there is someone trying to kill me the legal thing to do is to call the police and have them arrest the assailant. Self-defense (in Michigan anyway) is usually only allowed in imminent defense of life. I am not allowed to run after a robber or assailant and shoot them down and then claim self-defense. I am not allowed to shoot someone on the street because we had issues going back years. I can not shoot someone because of what they might do. That's logic for the non-criminal.

Criminal logic is of course quite different and can be very tempting/intoxicating when depicted fictionally on screen or in print but in real life we should remember that such an expanded claim of self-defense (pre-emption) could cause a lot more murders.

In prison if someone blows a kiss at me, stares at me too long, cuts in line for the phone or takes food from my plate it is entirely rational that I would then assault or attempt to kill them. To not do so would be to invite further abuse, rape or death. That would indeed be self-defense. But in "real life" that logic makes no sense and is totally destructive of any sort of society.

Michael initially did the wrong thing for the right reasons (or maybe the right thing for the wrong reasons) but whatever his good intentions may have been he changed from a straight laced college grad to a man who murdered his own brother, oversaw the degradation of hundreds of thousands via narcotics and prostitution and killed other men to take their goods for himself. This all may have been justified under the rules he was operating under but that doesn't make it "good" nor does it mean that he never had choices.