The standard of 'the strong equals justice' is a destructive standard. It is tyrannical. Any society based on this will inevitably fall apart and or destroy itself. The only way it could have any real longevity is if there is some kind of divine intervention to ensure that only good leaders and the like gain enough strength to impose their own 'justice.'
That's of course ignoring the fact that in reality it's always easier to kill someone than to withstand their attacks. ie- assassination, poisoning, etc. But hey, this is shounen manga, where dramatic arc determines if an attack will be successful, I get that.